Closes gh-1908

Previously, index.html used absolute paths to load its CSS and
JavaScript. This meant that it had to be deployed to /. This commit
updates the HTML to use relative paths for its CSS and JavaScript,
thereby ensuring that they can be loaded irrespective of the context
path to which the application is deployed.

Closes gh-1988

(cherry picked from commit d4bf221f)

Fixes a couple of typos in the documentation:
* `x-forwarded-protocol` is not the standard name for this header
* `require_https` is not an existing property, but rather `require_ssl`

Some environments do not have a tmpdir so it can be null.

Previously, if Logback was being used as Boot's logging system, but
Log4J was also on the classpath before SLF4J and Logback, JBoss
Logging would use Log4J for its logging. This lead to warning messages
being produced as Log4J was not configured:

log4j:WARN No appenders could be found for logger (org.jboss.logging).
log4j:WARN Please initialize the log4j system properly.

This commit updates LogbackLoggingSystem to set the
org.jboss.logging.provider to "slf4j". This ensure that JBoss Logging
will use SLF4J and Logback as intended even when Log4J is also on the
classpath.

Closes gh-1928

Add LICENSE.txt file and make it clear in the README file that we
are Apache 2.0

Fixes gh-1952

Fixes gh-1948

Closes gh-1919

Previously, only the http.mappers.json-sort-keys property was applied
by JacksonAutoConfiguration. This commit updates it to also apply the
http.mappers.json-pretty-print property as well.

See #1919

Closes gh-1936

Closes gh-1906

Closes gh-1905

Fixes gh-1944

Previously, for a non-async response with a successful status (< 400),
ErrorPageFilter would always call flushBuffer. This triggers an
exception in Tomcat if the client has closed the connection before the
response has been fully sent. In this case, Tomcat treats the response
as successful and commits it before control returns to the filter.

This commit updates ErrorPageFilter to only perform the flush if the
response has not already been committed, leaving any further flushing
that may be necessary to be handled by the servlet container.

Fixes gh-1938

Update ConfigurationPropertiesReportEndpoint to find
@ConfigurationProperties using `context.findAnnotationOnBean` rather
than `AnnotationUtils.findAnnotation`. This will correctly find the
annotation even if the bean is an interface based proxy.

Fixes gh-1927

Switch from external docbook XSLT to those shipped as part of the
Maven plugin.

Fixes gh-1942

Relax the servlet version check in Maven integration tests.

See gh-1797

Fixes gh-1797

Fixes gh-1874

Closes gh-1785

Closes gh-1893

The exception message for a connection timeout has been updated to
include the timeout period. The tests for the sample have been updated
accordingly.

Closes gh-1884

Closes gh-1883

Closes gh-1882

Closes gh-1881

Closes gh-1880

Closes gh-1879

Closes gh-1878

Closes gh-1877

Closes gh-1876

Closes gh-1875

Fixes gh-1712

Update MultipartAutoConfiguration to detect any MultipartResolver beans
rather than just StandardServletMultipartResolvers.

Fixes gh-1857