Filter out invalid completion entries and fix room avatars in the completion menu
When auto-completing rooms, there may be some that do not have canonical addresses (e.g. 1-to-1 chats). These can't be linked anyway, so skip them and any other unlinkable entries.
Room avatar URLs don't specify the new mxc:// format the user avatars use, so we need to carve out a workaround for them.