Added config.php option to replace the default implementation of system
tag manager and system tag object mapper.
Also adjusted the comments manager factory to inject the server container
we do not listen to deletion hooks anymore, because there is no guarantee that they
will be heard - requires that something fetches the CommentsManager first.
Instead, in the user deletion routine the clean up method will be called directly. Same way
as it happens for files, group memberships, config values.
register CommentsManager service, allow override, document in config.sample.php
don't insert autoincrement ids in tests, because of dislikes from oracle and pgsql
specify timezone in null date
only accepts strings for ID parameter that can be converted to int
replace forgotten hardcoded IDs in tests
react on deleted users
react on file deletion
Postgresql compatibility
lastInsertId needs *PREFIX* with the table name
do not listen for file deletion, because it is not reliable (trashbin, external storages)
add runtime cache for comments
