Fix the French translation for the add_back string

It was translated to something like “add again”, which doesn’t carry the same meaning and can be confusing for users.

It would be useful to also check how it has been translated in other languages, and add a source comment for future translators.

fixed crash on receiving files

* fix omemo in group chats w/o participants
* don't create two axolotl messages when messaging self
* fix read marker for self messages