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/components/suggestion/at_mention_provider.jsx

@@ -43,6 +43,15 @@ class AtMentionSuggestion extends Suggestion {
                 />
             );
             icon = <i className='mention__image fa fa-users fa-2x'/>;
+        } else if (user.username === 'here') {
+            username = 'here';
+            description = (
+                <FormattedMessage
+                    id='suggestion.mention.here'
+                    defaultMessage='Notifies everyone in the channel and online'
+                />
+            );
+            icon = <i className='mention__image fa fa-users fa-2x'/>;
         } else {
             username = user.username;
 
@@ -126,6 +135,9 @@ export default class AtMentionProvider {
                 if ('all'.startsWith(prefix)) {
                     filtered.push({username: 'all'});
                 }
+                if ('here'.startsWith(prefix)) {
+                    filtered.push({username: 'here'});
+                }
             }
 
             filtered.sort((a, b) => {