Consistently use singular for packages which contain mixed kinds of
classes. Moved a few classes to packages that better fit them.

This is used by RoomController to allow requests to /room/~{shortId}.

Classes can register reasons for maintenance. While at least one reason
is set, all requests are blocked and responded to with a 503 status
code.

Ektorp is introduced as a replacement for CouchDB4J.

At least caching of learning progress responses
did not work since commit a59eb75f on March, 6th.

While the exact origin of this problem is unclear,
it seemed like Spring was creating more than one
databaseDao bean. One of these beans had no
working caching annotations.

Events for all learning progress related actions
are generated, like answering a new question.
The event classes are used as a tag to evict
the cache based on the session.

Updated copyright year and adjusted header format to be consistent over
all files and with other ARSnova projects.