matrix: add mechanism for tab-completing/mentioning names in rooms
Most matrix/IRC/whatever clients support tab-completing nicks in a room/channel, so it's easy to mention folks in conversation.
The weechat-android client (weechat IRC client for android) adds a button in the text field area for tab completing, which might be a good way to implement this here in this random screenshot I found, it's the 2nd icon from the left, between the text input area and the paper airplane 'send' icon: