Refine Mustache support to provide a cleaner separation between the
reactive and servlet implementations. The views have now moved to the
`spring-boot` project and the auto-configuration has been split into
two distinct `@Imports` to save needing full package declarations.

See gh-8941

Closes gh-8996

Closes gh-8995

Closes gh-8994

Closes gh-8993

Closes gh-8992

Closes gh-8991

This commit simplifies the Mustache support for Spring MVC and removes
the included (view-based) i18n support in favor of more idiomatic
constructs like Mustache lambdas.

Fixes gh-8941

This commit moves the existing Spring MVC Mustache support to its own
`servlet` package and adds a new one under `reactive` for the WebFlux
web applications.

New `MustacheView` and `MustacheViewResolver` types resolve and render
Mustache views for WebFlux applications.

Since this templating engine is now supported by two flavors of Spring
web apps, the `spring-boot-starter-mustache` does not depend anymore on
the `spring-boot-starter-web` one: it's up to the developer to add the
relevant starter `web` or `webflux` to their application.

Fixes gh-8648

Closes gh-8964

Excluded the commons-logging dependency pulled in by Spring Security
until they can provide a fix.

See gh-8985

* pr/8881:
  Polish "Add database initializer for Spring Integration"
  Add database initializer for Spring Integration

Closes gh-8881

See gh-8881

* pr/8843:
  Polish "Support of Neo4j auto-index configuration"
  Support of Neo4j auto-index configuration

Closes gh-8843

See gh-8843

* pr/8758:
  Add Log4jdbc Starter reference

Clsoes gh-8758

Previously, the presence of a src/main/webapp/org directory would
cause the execution of BootWar to fail due to an attempt to write
a duplicate org/ entry to the zip output stream.

This commit updates BootZipCopyAction so that FileTreeElements that
match a directory entry created while writing the loader classes are
skipped.

Closes gh-8972

See gh-8968

Closes gh-8287

Rename WebFluxAnnotationAutoConfiguration to WebFluxAutoConfiguration
to better align with Spring MVC.

Remove the public constant to make it clearer that skipping path
extensions is really an internal Spring Boot concern.

See gh-8765

Replace the MVC validator post processor with an `@Import` so
that a "cannot enhance @Configuration bean definition"
warning does not occur.

Fixes gh-8951
See gh-8495

Closes gh-8961

Closes gh-8960

Closes gh-8959

Closes gh-8958

Closes gh-8957