core: Locale docs servlet.

- Added DocsServlet to automatically redirect users if
the servlet cannot find the documentation in the users locale.
- Added a check to make sure we are being passed a valid locale.
- Added an intermediate page to inform the user that the
documentation in their locale does not exist and how to install
it.
- Added unit tests for all used classes.

Change-Id: I37156061cbdd7d2df3290c88dee933c41e0087c5
Bug-Url: https://bugzilla.redhat.com/885823
Signed-off-by: Alexander Wels <awels@redhat.com>