"Reply" button for a sent message, sends a message to the request user

[ffont]

See the original bug report here: https://freesound.org/forum/bug-reports-errors-and-feature-requests/44525/?page=1#post102638
User A (the request.user) sends a message to user B.
User A wants to send a reply to the message before user B answers so user A navigates to the "sent messages" tab, opens the message and clicks "Reply". The reply form has user A's username pre-loaded, so the message will be sent to user A and not user B.

I guess the fix is to make sure that in any reply forms we always pre-fill the name of the "other" user in the conversation (the user who's not the `request.user).