add icommentsmanger and icomment implementation

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