fix bug where abstention questions are not exported

implement: via two booleans you can now decide whether AnswerStatistics and FeedbackQuestions shall be exported, too

refactored handling of logo during the saving of a new session and the update of an existing session out into a new method handleLogo

Retrieved @Cacheable and changed updateSession method in SessionService, now getting the session by keyword and set all fields that may have changed to a new value

This also fixes a test error.

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.

A NotFoundException was mistakenly thrown when the Session was set to
null via Socket.IO. Additionally, the user count was sent before it
had been updated.

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

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