Column opening/closing_on replaced with x/x_at date time attrs.

• Renames attributes closing_on to closing_at, and opening_on to opening_at
• Modifies date select in admin to include time
• Front-end now displays time in ordinal_long format
• Expiring consultation cache when they open / close
• Consultation scopes compare to Time.zone.now instead of Date.today
• Renames Consultation validation to validate_closes_after_opens
• Adds tests for PublicationesquePresenter#time_until_closure and changes it to handle times
• Modifies some other tests involving consultations to use times
Moved admin-only helpers into admin side

ConsultationsHelper#consultation_opening_phrase and consultation_closing_phrase were only being used in the admin suite.
They are now included in an admin helper.

Also updated the language to be 'Opens at' instead of 'Opens on'
Merge pull request #1007 from alphagov/make-edition-opening-on-closing-on-datetime

Tracker: https://www.pivotaltracker.com/story/show/59396028

Make edition opening on closing on datetime