fix most recent post stolen by the last logged in user AND only show active users in post-settings-menu drop-down

fixes #3549 fixes #3528
- instead of grabbing and setting the author after fetching the list of users, get the author info from the models "author" property and set it when that value returns. (3549)
- added filter to the PSM's "authors" attribute to only display active users (3528)
Autofocus on input fields after routing

closes #3538, closes #3541
- The “autofocus” property isn’t considered by browsers if the element
is created after the page has been loaded. The Ember cookbook
recommends extending the input type with a manually triggered
autofocus, which I included here.
- This introduces the new helper “gh-focus-input”, an input field
gaining focus even when routing occurs.
Merge pull request #3581 from felixrieseberg/iss3541

Autofocus on input fields after routing
Merge pull request #3569 from morficus/issue-3549

fix most recent post stolen by the last logged in user AND only show active users in post-settings-menu drop-down
Add classes to settings headers

No issue

Refs https://github.com/TryGhost/Ghost-UI/issues/91

- Adds a class to settings page headers, for cleaner CSS selectors