Aggregated documentation setup

Initial setup for aggregated documentation across modules ([omnidoc](https://github.com/playframework/omnidoc)).

- changes to the docs plugin for running from external modules (mainly moving things to the docs plugin)
- add omnidoc support (publish playdoc jar and include source url for linking)
- remove twirl docs (added to twirl here: https://github.com/playframework/twirl/pull/46)
- add placeholder entry for play-slick docs
- make the documentation jar optional and configurable
- expect play-omnidoc to be published for released versions

The existing play-docs jar and publishing is left as is, and is used as a fallback for when play-omnidoc is not expected or disabled.

Omnidoc is likely to be a temporary arrangement, with a properly modular documentation setup in the future.