Some bell notifications need to be removed at a specific time without
an event happening at this specific time. Others need to be updated
then, as attributes change, for example their count.

This commit implements a call in the listBells() method of the
BellGateway class, that calls the newly introduced BellUpdateTrigger to
update expired bells before they are delivered to the client.

This implementation stands in contrast to our immediate bell updates via
the webSocket, but the expiration updates would require a cron job or
a tricky client side implementation to be triggered immediately, that's
why it's triggered every time bells are fetched from the database now,
which means that it will happen after every page reloading and after
anything else that triggers an update of the bells.

The BellUpdateTrigger is a service, that all classes that need to update
their bells when they expire, should subscribe to. For that, they need to
implement the updateExpiredBells function of the BellUpdaterInterface.
After they subscribed, their implementations of the updateExpiredBells()
function will be called every time the someone calls the
BellUpdateTrigger::triggerUpdate() function (which, as already
mentioned, currently happens before bells are fetched from the
database).

This implementation allows to easily change the trigger for
the updates, for example, we could easily move it to a cronjob or to an
Xhr function that gets called by the frontend. It also makes it easily
possible for every class to become a bell updater and send bells with
an expiration date to update them later.