[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/utils/utils.jsx

@@ -989,12 +989,16 @@ export function changeOpacity(oldColor, opacity) {
 }
 
 export function getFullName(user) {
-    if (user.first_name && user.last_name) {
-        return user.first_name + ' ' + user.last_name;
-    } else if (user.first_name) {
-        return user.first_name;
-    } else if (user.last_name) {
-        return user.last_name;
+    if (user !== null && typeof user !== 'undefined' && typeof user === 'object') {
+        const firstName = user.hasOwnProperty('first_name') ? user.first_name : '';
+        const lastName = user.hasOwnProperty('last_name') ? user.last_name : '';
+        if (firstName && lastName) {
+            return firstName + ' ' + lastName;
+        } else if (firstName) {
+            return firstName;
+        } else if (lastName) {
+            return lastName;
+        }
     }
 
     return '';