PLT-914 Add mention notifications for replies on a comment thread (#3130)

* PLT-914 Add mention notifications for replies on a comment thread

* remove useless store method
fix highlighting comments posted before th user write something to thread

* refactor out isCommentMention function after rebase

* change comment bar highlighting to replay icon mention highlighting

* settings and always visible highlight

* fix unit tests for new settings

* change highlight behaviour
- if any message in comment thread generates mention - all thread is highlighted
- remove always visible highlightion

* fix bug about the textarea in the center channel not clearing

* fix default settings value notify_props.comments

* do not highlight own comments if there is no other user's messages in thread

* refactor out ReactDOM.findDOMNode

* refactor out using of UserStore from component

webapp/sass/layout/_post.scss

@@ -560,7 +560,7 @@ body.ios {
             .img-div {
                 max-height: 150px;
                 max-width: 150px;
-            }
+        }
 
             p {
                 line-height: inherit;
@@ -572,7 +572,7 @@ body.ios {
 
             ol,
             ul {
-                clear: both;
+            clear: both;
                 padding-left: 20px;
             }
         }
@@ -1070,7 +1070,6 @@ body.ios {
         display: inline-block;
         margin-right: 6px;
         visibility: hidden;
-
         svg {
             fill: inherit;
             position: relative;