[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/more_direct_channels/more_direct_channels.jsx

@@ -2,7 +2,7 @@
 // See License.txt for license information.
 
 import MultiSelect from 'components/multiselect/multiselect.jsx';
-import ProfilePicture from 'components/profile_picture.jsx';
+import ProfilePicture from 'components/profile_popover/picture_profile_popover.jsx';
 
 import {searchUsers} from 'actions/user_actions.jsx';
 import {openDirectChannelToUser, openGroupChannelToUsers} from 'actions/channel_actions.jsx';