[PLT-7342] Add function and tests to specifically determine @all & @channel (#7181)

* add function and tests to specifically determine @all & @channel

* uodate per comments - regex and tests

webapp/components/create_post.jsx

@@ -14,6 +14,7 @@ import * as EmojiPicker from 'components/emoji_picker/emoji_picker.jsx';
 import AppDispatcher from 'dispatcher/app_dispatcher.jsx';
 import * as GlobalActions from 'actions/global_actions.jsx';
 import * as Utils from 'utils/utils.jsx';
+import * as PostUtils from 'utils/post_utils.jsx';
 import * as UserAgent from 'utils/user_agent.jsx';
 import * as ChannelActions from 'actions/channel_actions.jsx';
 import * as PostActions from 'actions/post_actions.jsx';
@@ -224,7 +225,7 @@ export default class CreatePost extends React.Component {
         const members = stats.member_count - 1;
         const updateChannel = ChannelStore.getCurrent();
 
-        if ((this.state.message.includes('@all') || this.state.message.includes('@channel')) && members >= Constants.NOTIFY_ALL_MEMBERS) {
+        if ((PostUtils.containsAtMention(this.state.message, '@all') || PostUtils.containsAtMention(this.state.message, '@channel')) && members >= Constants.NOTIFY_ALL_MEMBERS) {
             this.setState({totalMembers: members});
             this.showNotifyAllModal();
             return;