Bump govuk_content_models to 4.5.0

This adds `alert_status` and `summary` fields.
Add alert statuses to edit form

Text for statuses are stored in the locale file
and looked up from the `ALERT_STATUSES` array
in `TravelAdviceEdition`.
Strip out any empty alert statuses on update

Formtastic inserts an empty value into the checkbox
array as a hidden field, so that when none are
checked, the field is still submitted as an empty
array.

This breaks our validation on the array values, so
here we remove any before the action logic begins.
Feature spec for alert status checkboxes.

Additional test for making sure unchecking all
the boxes will save.
Use blank? check for alert_status field
Combine alert status checkbox feature specs
Merge remote branch 'origin/master' into alert-status

Conflicts:
	app/helpers/application_helper.rb
	app/views/admin/editions/edit.html.erb
Merge remote branch 'origin/alert-status'