PLT-2408 Adds here mention for online users (#3619)

* Added @here mention that notifies online users

* Fixed existing race condition that would sometime cause clients to miss mention count changes

* Added missing localization strings

* Prevent @here from mentioning the user who posted it

webapp/utils/text_formatting.jsx

@@ -207,6 +207,7 @@ function highlightCurrentMentions(text, tokens) {
     let output = text;
 
     const mentionKeys = UserStore.getCurrentMentionKeys();
+    mentionKeys.push('@here');
 
     // look for any existing tokens which are self mentions and should be highlighted
     var newTokens = new Map();