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"
Merge pull request #2426 from rhodri/patch-1

Fix incorrect regex