Add alert_status array to travel advice edition

The alert status can contain any of a set of predefined values.
Test the alert_status value in the has_correct_fields test
Remove the no restrictions alert status

It's not an alert status, as such - 'no restrictions', in theory,
could be inferred in code by there not being any alert statuses
set.
Add summary field to travel-advice-publisher
Merge pull request #49 from alphagov/travel-advice-alert-status

Add alert_status array to travel advice edition