Disjointed popovers

• Separates popovers from DOM hierarchy by teleporting them to a separate container
• Uses position: fixed to position popovers. As a result, popovers are now forcibly hidden on scroll
• Recaclculates popover position on scroll event. Works like a charm on desktop (sub 1ms per scroll update), a bit laggy but acceptable on firefox mobile, popover vibrates but follows the scroll on mobile chrome.
• Fixes duplicate <button> elements on react/extra buttons
• Uses vue transition to fix animations on popups/tooltips
• Fixes shadows so that all popovers/tooltips have proper shadows, not just status popovers
• Replaces ugly bootleg tooltips for mention links with floating usercards (on click)

Known issues

• hide-on-scroll only works on main document scroll, not on column scroll. Now searches for nearby .column.-scrollable and falls back to using window otherwise
• tests are likely fucked due to changes in mentionlinks stuff? yep. fucked. fixed now
• performance needs testing/verification
  • when i get browserstack i can test ios, i expect it to be jank af but workable.
    • actually, performance is stellar, at least best i could tell, browserstack itself is pretty laggy tho, can't test thoroughly
  • maybe add advanced option to not do that in case it causes problems?
• ❔ maybe it's too easy now to keep hover popover open?
• ❔ popover position doesn't update when timeline moves, but wiring that into vuex/vue events might have performance consequences