Prior to this commit, the devtools used bean factory post processors to
configure the environment with custom, development-time properties. This
meant that the environment was configured as part of the application
context being refreshed. Crucially, this happened after any property
conditions were evaluated making it impossible for the devtools to
change the default auto-configuration behaviour for a bean or
configuration class that was conditional on a property.

This commit moves the configuration of the environment into an
ApplicationListener that listens for the
ApplicationEnvironmentPreparedEvent which is published as soon as the
Environment has been prepared and before the application context is
refreshed.

Closes gh-3726

The commit introduces a new extension point, EnvironmentPostProcessor,
that can be implemented by classes that want to modify the
environment. Implementations of EnvironmentPostProcessor are loaded
via spring.factories and called in response to the
ApplicationEnvironmentPreparedEvent. Application listeners that wish
to work with the post-processed environment can continue to listen
to ApplicationEnvironmentPreparedEvent and order themselves to
run after EnvironmentPostProcessingApplicationListener.

Existing ApplicationListeners that modify the environment have,
where possible, been updated to implement EnvironmentPostProcessor
instead.

Closes gh-3737

Conflicts:
	spring-boot-actuator/src/main/java/org/springframework/boot/actuate/autoconfigure/HealthIndicatorAutoConfiguration.java

The DataSource health indicator uses `JdbcTemplate` behind the scenes
but nothing was checking that it is actually available.

`DataSourcesHealthIndicatorConfiguration` is now disabled if
`spring-jdbc` is not on the classpath.

Fixes gh-3765

* pr/3721:
  Fix typo

Closes gh-3721

* pr/3752:
  Fix table layout

Closes gh-3752

Closes gh-3760

Closes gh-3751

Closes gh-3756

On start.spring.io, if you customize the artifactId it creates a zip file
with the same name. The `spring init` command did not have a similar
shortcut.

This commit updates the request to customize the artifactId if none is
set and a custom location was specified. Just as we check for the
presence of a dot to figure out if we have to extract the archive or not,
we check for it to generate an artifactId without an extension.

In practice, `spring init foo` creates a foo directory with a project
whose artifactId is `foo` and `spring init foo.zip` stores a foo.zip
file with the same project (i.e. the artifactId is `foo`).

Closes gh-3714

The `spring.datasource.name` property was hidden behind the 'name'
attribute of the Tomcat connection pool (since we are mapping all
datasource implementations on the `spring.datasource` namespace.

This commit replace the injected value by hand with the use of the
regular `DataSourceProperties`. That way, we generate proper meta-data
for it as well.

Closes gh-3755

A new `spring.datasource.type` property can now be used to configure the
connection pool implementation to use (rather than only relying on Boot's
preferences).

Closes gh-3705

Improve the documentation to explain the necessary steps to create a
custom Spring Boot starter. In particular, provide more details regarding
naming conventions.

Closes gh-2537
See gh-2927

* pr/3750:
  Polish
  Add fallbackToSystemLocale configuration key

Closes gh-3750

Allow to configure the `fallbackToSystemLocale` attribute of
`MessageSource` via configuration.

Closes gh-3749

Previously, to use a custom ConfigurableWebBindingInitializer, it was
necessary to extend WebMvcConfigurationSupport and override
getConfigurableWebBindingInitializer. This had the unwanted
side-effect of switching off the auto-configuration of Spring MVC.

This commit updates the auto-configuration to look for a
ConfigurableWebBindingInitializer bean and register it with Spring
MVC.

Closes gh-2526

Closes gh-3516

Add:

 - jackson-datatype-hibernate4
 - jackson-datatype-hibernate5
 - jackson-datatype-jdk7
 - jackson-module-parameter-names

Closes gh-3727

Closes gh-3738

Previously, PropertiesConfigurationFactory would only create a
DefaultPropertyNamePatternsMatcher that ignored case if it was
ignoring unknown fields. If the binding had a target name and
unknown fields were not being ignored the matcher would consider the
case when finding matches. This meant that SERVER_PORT would not being
to ServerProperties.port as SERVER did not match the target name,
server.

This commit updates PropertiesConfigurationFactory to use a
case-ignoring DefaultPropertyNamePatternsMatcher when binding with
a target name. The tests have also been updated to test binding both
with and without ignoring case and using either Properties or
PropertySources. The above-described matching is only performed
against PropertySources and the bug only occurred when using a target
name, making it important to test all combinations.

Closes gh-3745

The target attribute is effectively checked for null beforehand so this
additional defensive check can be removed.

Closes gh-3725

See gh-3696

Closes gh-3423

This commit adds support for using the environment to configure the
Freemarker and Velocity view resolvers to allow session overrides.

Closes gh-3410