Add a reviewed_at method to edition controller

This updates the current edition's reviewed_at field to be the current
time
Bump content_models gem to 4.19
Add test to assert reviewed_at is correctly updated
Remove old :reviewed_at reference
Redirect to the country edition view page when saving reviewed_at
Change update review button to green
Refactor conditional in view to ternary
Merge remote-tracking branch 'origin/add_reviewed_at_field'