All feature settings are replaced by the received object so the use of
PATCH does not make sense here. The session is "patched" but the HTTP
verb always relates to the URL which refers explicitly to features in
this case.

See https://tools.ietf.org/html/rfc5789.

This disables CORS for this path. It should not be usable by external
websites.

This prevents DoS attacks caused by request loops.

Renamed optional.arsnova to ui.arsnova-slogan and made comments start
with an uppercase letter.

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

Removed @Chacheable from getSession function, because the Update of a session wont work if only the cache is used for loading the content of a session

Requesting partial lists would interfere with Spring caching so
pagination is handled by the newly introduced PaginationListDecorator
in the controllers. This additionally allows to determine the total
count of items (independent from pagination parameters) which is not
possible with CouchDB.

The mentioned method is also changed to only retrieve http header
informations instead of whole URI content.

Added TODOs to keep track of methods still evicting the whole answers
cache for future improvements. Additional refactoring is necessary to
implement these.