Req: Interpret mentions from Hangouts users

[bobpaul]

While hangouts doesn't support mentions (and it's not possible to support mentioning hangouts users from matrix), it would be nice if matrix users could be mentioned by hangouts users.

I think @localpart for users on the homeserver or @full:matrix.id for any users would be good to have. I think the latter actually works (though when a friend and I tested, I didn't get an audible alert).

This would be really nice for some extra chat user/groups, as it would allow turning notifications off for that room but still give hangouts users a way to alert if something important happens.