Move external source link to extra buttons, expand the expand button

Motivation: reduces visual clutter, makes you feel less like a bomb defuser when expanding conversations on mobile (now there's actually nothing else interactive next to expand button in normal situation so I expanded its hitbox too). The external source link is not important enough to warrant always-visible button now that we're not on GS anymore where conversation pulling was abysmal.

Also some minor code cleanup and improvement: the heading row buttons now highlight on hover

src/components/extra_buttons/extra_buttons.js

@@ -5,7 +5,8 @@ import {
   faBookmark,
   faEyeSlash,
   faThumbtack,
-  faShareAlt
+  faShareAlt,
+  faExternalLinkAlt
 } from '@fortawesome/free-solid-svg-icons'
 import {
   faBookmark as faBookmarkReg
@@ -17,7 +18,8 @@ library.add(
   faBookmarkReg,
   faEyeSlash,
   faThumbtack,
-  faShareAlt
+  faShareAlt,
+  faExternalLinkAlt
 )
 
 const ExtraButtons = {