[Security] Escape nicknames starting with a forward slash `/` in mentions

So you can't be tricked into sending malicious commands. Related: #658 for making it possible to escape the slash in the first place.

[ara4n]

@bmarty i think this issue has slipped through the cracks - it should be a trivial fix; can you have control please? :)

[bmarty]

Ok, fixed in Riot 0.8.15