Utilize modifiedSince parameter for room updates

[SystemKeeper]

Currently we only use modifiedSince for updates in the background fetch, but never for the actually room update. I assume this was never implemented, as we currently do not handle delete/disinvite signaling messages but instead always do a full fetch. We should consider to also use modifiedSince in normal operations and add support for delete/disinvite signaling messages.

Docs https://nextcloud-talk.readthedocs.io/en/latest/conversation/#get-user-s-conversations