'Publish' acts as 'Save & Publish'

When users hit 'Publish' it should save and then attempt to publish
the edition in question.
Update preview button copy text

Make it explicit that we're previewing the last saved version.
Reword test scenario to be easier to understand
Don't show 'Save & Publish' button for archived and published editions
Redo controller logic nesting to be more defensive

Handle the case where a publish_as call fails so we show something to
the user.
Merge remote branch 'origin/master' into save-and-publish-preview
Fix tests to work with new model validations
Remove now-redundant route
Merge remote branch 'origin/save-and-publish-preview'