This commit configures Gradle build scans for the Spring Framework
build. Scans aren't performed for every build but can be enabled
with the following:

```
./gradlew build --scan
```

Issue: SPR-15885

Closes gh-1501

Includes general streamlining of dependency declarations with reduced version variables, direct use of EclipseLink 2.7 and its implicit JPA 2.2 dependency in spring-orm, mixed use of Hibernate 5.2.10 and 5.1.10 for integration tests, as well as an upgrade to Jetty 9.4.7 RC0 and a downgrade to Groovy 2.4.12 (since Groovy 2.5 won't be final in time for Spring Framework 5.0).

Issue: SPR-15879
Issue: SPR-15880

Issue: SPR-15673

Issue: SPR-15149

Issue: SPR-15871

Issue: SPR-15872

Issue: SPR-15869

Issue: SPR-15532

Bean-derived null values may still get passed into bean properties and injection points but only if those are declared as non-required. Note that getBean will never return null; a manual bean.equals(null) / "null".equals(bean.toString()) check identifies expected null values now.  This will only ever happen with custom FactoryBeans or factory methods returning null - and since all common cases are handled by autowiring or bean property values in bean definitions, there should be no need to ever manually check for such a null value received from getBean.

Issue: SPR-15829

Issue: SPR-15149

Refer to both Servlet and reactive stacks in web.adoc
Move web-integration.adoc to the very end
Link in web-flux-functional.adoc
Insert "Spring Web MVC" or "Servlet" prefix in chapter titles

Issue: SPR-15149

Issue: SPR-15869

Includes latest dependency updates (Tomcat 8.5.20, Protobuf 3.4, RxJava 2.1.3, Caffeine 2.5.5)

Issue: SPR-15870

Issue: SPR-15858

Issue: SPR-15542

Issue: SPR-15867

Instead of always using the default JSON mime types

Issue: SPR-15860

Issue: SPR-15852

This commit restores and existing redirectedUrl and forwardedUrl
methods (simple String comparison) and creates separate
redirectedUrlTemplate and forwardedUrlTemplate methods that expand
and also encode before comparing.

Issue: SPR-15834

Issue: SPR-15819

Issue: SPR-15863

Issue: SPR-15814

Issue: SPR-15851

This commit also fixes compilation with JDK9 since
KT-18832 has been fixed.

Issue: SPR-15814

Issue: SPR-15841