Deprecate and stop using the overflowingExecutionContext

Fixes #2947.

The overflowingExecutionContext is tricky to use safely so
this change removes all uses from Play. The object has been
deprecated for Play 2.3 and should be removed in Play 2.4.

In all cases the trampoline ExecutionContext should be used
instead since it is much safer and offers basically the same
performance.
Remove overflowingExecutionContext

The overflowingExecutionContext was deprecated for Play 2.3 and can
be removed in Play 2.4.
Update to twirl 1.0.0
Fix hostnameVerifier using a config Option[Class]
Use Play.application.classloader to load hostnameVerifierClass, change parser to use "ws.ssl.hostnameVerifierClass" and change SSLConfig to take a class instead of a String.
Expose play.libs.Json.mapper to make custom Json manipulation easier
Merge pull request #2953 from pvlugter/twirl-1.0

Update to twirl 1.0.0
Merge pull request #2952 from richdougherty/no-overflow

Deprecate/remove the overflowingExecutionContext
Merge pull request #2958 from gmethvin/mapper

Expose play.libs.Json.mapper to make custom Json manipulation easier
Merge pull request #2957 from wsargent/fix-2779-master

[2.4/master] Change hostname verifier to use thread's classloader
Add bold styling to "NOTE"
Update ebean doc
Merge pull request #2426 from rhodri/patch-1

Fix incorrect regex
update typesafe config 1.2.1

akka 2.3.3 depends on typesafe config 1.2.1 https://github.com/akka/akka/blob/v2.3.3/project/AkkaBuild.scala#L1146
I think we should use same version
Lazily load SecurityHeadersConfig
Merge pull request #2974 from gmethvin/security_headers

Lazily load SecurityHeadersConfig
Fix ScalaDependencyInjection.md's Spring description grammar
Update SBT Cookbook documentation.

The SBT Cookbook contained a reference to the playAssetsDirectories
attribute that was removed in play 2.3.
Added details of how to import the cache into the documentation
Merge pull request #2990 from Lykathia/documentation-update

[doc] Update SBT Cookbook documentation.
Merge pull request #2982 from leeduan/master

Fix ScalaDependencyInjection.md's Spring description grammar