improve channel history / remove channel subscription

[abma]

-> let the lobby client sent what history it wants:

• lobby server doesn't need to store a date
• no gaps when client crashed / timed out

ideally by last msg id and not by time

[abma]

historical data should contain timestamps (which are already in db, so the required change would be minor)

[gajop]

-> let the lobby client sent what history it wants:
+1, and ideally also allow the lobby to retrieve historical data that has already been sent (keep history).
This is pretty useful when switching PCs/lobbies without having to rely on local backups.

[abma]

todo:

1. implement client side
2. remove the old implementation (especially the check in SQLUsers.py / get_channel_history for subscription)
3. remove subscription table / commands

[silentwings]

rel #312 #241

[silentwings]

as far as i can see, all this has already been done