Document the removal of implicit parameters

Fixes #2799

Documented the removal of the session, flash and lang implicit
parameters in the migration guide.
Deduplicate assets

Fixes #2936

If two modules depend on the same webjar, there will be duplicate
mappings to the assets in that webjar.  This commit fixes that by just
taking the first.
Documentation for thread locals and thread context class loaders
Merge pull request #2799 from jroper/2799-document-session-flash-removal

Document removal of session and flash implicit conversions
Merge pull request #2936 from jroper/2936-deduplicate-assets

[2.3] ZipException: duplicate entry while doing sbt 'dist'
Merge pull request #3071 from richdougherty/iteratee-thread-pool-doc2

Documentation for thread locals and thread context class loaders
Upgrade to Twirl 1.0.2
Merge pull request #3072 from benmccann/twirl102-take2

Upgrade to Twirl 1.0.2