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