Option to restrict hard-deleting messages in p2p topics to sender only #940

Add ability to restrict message deletion for both parties in p2p topics to message sender only.

For example, in a conversation between Alice and Bob, allow Bob to hard-delete messages sent by him for both Alice and himself. At the same time Bob won't be able to hard-delete messages sent by Alice.

This is how it works in WhatsApp.

Expanded from #249.