Usernames that contained underscores were parsed by markdown first. This
broke the diaspora IDs and also added weird html at places where it
wasn't needed. Escaping them before sending the message through the
markdown parser fixes this issue.
As underscores are the only allowed character that can be used for
markdown that is also allowed inside a diaspora ID, this escaping can be
kept pretty simple.
This only fixes it for the mobile UI at the moment, for the desktop UI
it's probably better to fix it in markdown-it.
Related to #7975
536c96f217