Automatically hard-delete messages in any topic by timer.

[or-else]

Add ability to expire (hard-delete) messages in any topic by timer.

• Topic owner can set expiration time for a group topic.
• Either party can set expiration time for a p2p topic.
• Allow site admin to set timer restrictions:
  • Enable/disable the feature site-wide.
  • Force message deletion site-wise.

Expanded from #249