[FIX] mail: composer: correctly erase mentions

The problem was that mentions weren't correctly erased with DELETE key (in some
conditions), or when replacing mention text. When this happened, the mention
didn't appear in the message, but the message was even tough sent to the erased
mentioned people.

With this rev., we don't try to detect erasing on the fly anymore, but we
rather validate mentions before sending the message with the content of the
input.

opw 669362