[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/package.json

@@ -15,6 +15,7 @@
     "flux": "3.1.2",
     "font-awesome": "4.7.0",
     "highlight.js": "9.10.0",
+    "html-to-react": "1.2.7",
     "inobounce": "0.1.4",
     "intl": "1.2.5",
     "jasny-bootstrap": "3.1.3",