getSession throws an HTTP exception, which is not
good style. Additionally, as it calls
getSessionFromKeyword internally -- which has a caching
annotation -- this caching mechanism will not work
because it only works if the method is called
from another object. This is due to the way Spring
is using proxies to intercept the calls.

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.

This change also removes the "lockSession" method from CouchDB.
Instead, the more general update method is used which also
has the required caching annotations.

Note that due to the way it is currently implemented,
we have to clear the whole cache once a set of
questions published or unpublished in bulk.

Added few pp Attributes to SessionInfo. Requests for Public Pool Sessions now return SessionInfo instead of Session.

databaseDao. Task #14478

count to NewAnswerEvent websocket broadcast. Task #14471

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

preperation questions. Task #14343