FIX: fix @mentions for usernames shorter than 2 characters

Super short usernames were enabled by
https://github.com/discourse/discourse/pull/2838
This is just a fix for @mentions

app/assets/javascripts/discourse/dialects/mention_dialect.js

@@ -7,7 +7,7 @@ Discourse.Dialect.inlineRegexp({
   start: '@',
   // NOTE: we really should be using SiteSettings here, but it loads later in process
   // also, if we do, we must ensure serverside version works as well
-  matcher: /^(@[A-Za-z0-9][A-Za-z0-9_]{1,40})/m,
+  matcher: /^(@[A-Za-z0-9][A-Za-z0-9_]{0,40})/m,
   wordBoundary: true,
 
   emitter: function(matches) {