assets:precompile: do not reset the whole cache.

By default, on rake assets:precompile, the whole cache is deleted. But to re-create the cache takes several hours. We cannot afford this on each deployment since we deploy several times a day.

Therefore, we override the precompile behaviour to clear only tmp/cache/assets.

References:
* http://www.reinteractive.net/posts/116-12-tips-for-the-rails-asset-pipeline
* https://github.com/rails/sprockets-rails
* https://github.com/rails/rails/blob/v3.2.3/actionpack/lib/sprockets/assets.rake\#L69

Trello: https://trello.com/c/gqhbW3qq/646-performance-optimierung-von-gruppen
Changing the location of the file_store cache to tmp/app_cache. (Before: app/cache.)

We do this, because the assets cache, by default, is also stored in tmp/cache and is reset on each deploy by rake assets:precompile. But, we do not want to delete the application cache on each deployment.

See also this previous commit:
https://github.com/fiedl/wingolfsplattform/commit/224b8c1ffcb01a8c452f3166e915903b796107c9

Trello: https://trello.com/c/gqhbW3qq/646-performance-optimierung-von-gruppen