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/constants.jsx

@@ -217,7 +217,7 @@ export const Constants = {
         ONLINE: 'online'
     },
 
-    SPECIAL_MENTIONS: ['all', 'channel'],
+    SPECIAL_MENTIONS: ['all', 'channel', 'here'],
     CHARACTER_LIMIT: 4000,
     IMAGE_TYPES: ['jpg', 'gif', 'bmp', 'png', 'jpeg'],
     AUDIO_TYPES: ['mp3', 'wav', 'wma', 'm4a', 'flac', 'aac', 'ogg'],

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();