Thymeleaf 3.0 implements the Spring 5.0 view infrastructure for WebMVC
and the new WebFlux framework. This commit adds auto-configuration for
the WebFlux support.

In that process, the configuration property for `spring.thymeleaf` has
been changed to add `spring.thymeleaf.servlet` and
`spring.thymeleaf.reactive` for MVC/WebFlux specific properties.

Now that the `spring-boot-starter-thymeleaf` does not only support
Spring MVC, the transitive dependency on `spring-boot-starter-web` is
removed from it.

Fixes gh-8124

* pr/9020:
  Upgrade to Spring Kafka 2.0.0.M1

Closes gh-9020

See gh-9000

Add a test to ensure that the new binder can bind correctly to a
Set.

Closes gh-1415

Closes gh-9000

Remove the deprecated relaxed binder code that has now been replaced
with the configuration properties binder.

Closes gh-9000

Add a test to ensure that the new binder can bind correctly to a
Map<String,String[]>.

Closes gh-3789

Add a test to ensure that the updated configuration properties work
correctly binds environment sources in the `/info` endpoint.

Closes gh-7388

Add a test case to show that the updated configuration properties binder
correctly binds environment variables to complex maps.

Closes gh-8902

A test to ensure that the updated configuration properties support
correctly expands placeholders using mapped properties.

Prior to the updated support an `application.properties` file of the
form:

	my.prefix.some-url=http://github.com
	my.prefix.login-url=${my.prefix.some-url}/login

Would not correctly expand the placeholder against the environment
variable `MY_PREFIX_SOME_URL`

Closes gh-2496

Remove the `relaxed` attribute from `@ConditionalOnProperty` and instead
rely on the direct configuration property source relaxed name support.

Closes gh-9003

Extend `BindFailureAnalyzer` to support the exceptions thrown by the new
configuration properties `Binder`. Origin information is now also
reported when available.

Closes gh-8934

Update `spring-boot-samples` to use the new configuration properties
support.

See gh-9000

Update `spring-boot-devtools` to use the new configuration properties
support.

See gh-9000

Update `spring-boot-actuator` to use the new configuration properties
support.

See gh-9000

Update `spring-boot-autoconfigure` to use the new configuration
properties support.

See gh-9000

Update `spring-boot-test` to use the new configuration properties
support.

See gh-9000

Update `spring-boot` to use the new configuration properties support.

See gh-9000

Remove `ConfigurationProperySource` from `PropertySourcesPropertyValues`
so that the deprecated code to continues to work.

See gh-4910

Update `SpringApplication` to automatically attached the
`ConfigurationPropertiesSource` when a `SpringApplication` runs.

See gh-4910

Create a new `Binder` class specifically designed to bind properties
from one or more `ConfigurationPropertySources` to an object.

The binder provides a replacement for `RelaxedBinder` and attempts to
fix the limitations of the previous solution.

Closes gh-8868

Add a new abstraction that represents a source for configuration
properties. The new source is similar to the `Environment` abstraction
provided by Spring Framework but follows a stricter set of rules.

The `ConfigurationPropertySource` provides a uniform view onto a source
and will help to move responsibility for accessing properties in a
"relaxed" way away from the caller.

The `ConfigurationPropertyName` class enforces strict naming rules
that callers must follow when accessing properties. Configuration
names are lowercase, dot separated and may contain dashes. In
addition "indexed" name elements may be defined by using square
brackets.

Mapping is provided to existing Spring PropertySources implemented with
the relaxed rules that users are used to. For example the configuration
property `server.local-host` can still be written in a property files
as `server.localHost` or in an environment variable as SERVER_LOCALHOST.

Closes gh-4910

Move the `Origin` and related classes from `o.s.boot.env` to
`o.s.boot.orgin` and extend support. The concept of an origin can now
be used outside of just the Spring Environment abstraction.

Closes gh-9001

Deprecate `RelaxedDataBinder`, `RelaxedPropertyResolver` and related
classes in preparation for the updated configuration properties binder
work.

See gh-9000

Update `SpringApplication` so that the `sources` property is
independent to the primary source provided on construction.

Prior to this commit the sources property was a little unusual in that
calling set would add to the existing collection.

Closes gh-8910

See gh-9019

Drop `spring-social-web-thymeleaf3` dependency since it's been merged
into `spring-social-web` as of Spring Social 2.0.

Fixes gh-9017

* pr/9011:
  Remove Spring Session GemFire dependency
  Remove Spring Session Mongo support

Remove Spring Session GemFire dependency since it is
no longer supported in Spring Session 2.0.

Closes gh-9011

Remove auto-configuration support for Spring Session Mongo since it is
no longer supported in Spring Session 2.0.

See gh-9011

Closes gh-6013