[GH-5915] Clicking on @mention of a user in a post shows the profile popover (#6129)

* post message at-mention profile pop-over

* remove hover effect to special mentions

* make non-clickable the non-existing at-mention username

* fix "video call" position

* use usernameMap instead of initially defined liteUsernameMap

* update per comments

webapp/components/user_list_row.jsx

@@ -1,7 +1,7 @@
 // Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
 // See License.txt for license information.
 
-import ProfilePicture from 'components/profile_picture.jsx';
+import ProfilePicture from 'components/profile_popover/picture_profile_popover.jsx';
 
 import UserStore from 'stores/user_store.jsx';
 import PreferenceStore from 'stores/preference_store.jsx';