Added popover component

Closes #2418, #2714
Ref #2446, #2565

- Added and injected `popover` service to globally control popovers
- Added `gh-popover-button` component
- Added `popover-mixin` for popover and popover-buttons to mixin
- Added body-event-listener mixin for popover service to watch for body
  clicks with
- Post settings and post save button both now use `gh-popover`
- Added hacks to `ember-hacks.css` to make popovers work until ghost-ui
  consolidates functionality
Merge pull request #2840 from novaugust/popover-service

Added popover component
Added error link on 404 pages

closes #2298
- added hyperlink to error templates
- added class for styling hyperlinks on error pages