Shorter user slugs (if possible)

Closes #4211
Fix up labels for controls in post settings menu

No issue
Merge pull request #4288 from jaswilli/psm-labels

Fix up labels for controls in post settings menu
Merge pull request #4220 from felixrieseberg/iss4211

Shorter user slugs (if possible)
Merge branch 'master' of https://github.com/TryGhost/Ghost

Conflicts:
	core/client/templates/post-settings-menu.hbs