Fixing long usernames (#2821)

* Submitting a fix for popover titles

* Fixing heading for channels

* Fixing other minor corner cases for long username

webapp/sass/components/_popover.scss

@@ -18,7 +18,10 @@
 
     .popover-title {
         background: alpha-color($black, .05);
+        max-width: 100%;
+        overflow: hidden;
         padding: 8px 10px;
+        text-overflow: ellipsis;
     }
 
     .popover-content {