Feed & email signup buttons always on same line
relevant_to_local_government support in feeds

• FeedUrlBuilder will build urls including the relevant_to_local_government param
• DocumentFilter::Description correctly describes feeds which are relevant to local government.
SubscriptionUrlGenerator handles relevant_to_local_government for every feed type
Move DocumentFilter::Description -> GovUkDelivery::EmailSignupDescription
Tidy up SubscriptionUrlGenerator code
Merge pull request #1241 from alphagov/notifications-for-people-and-roles-fixes

Notifications for people and roles fixes

https://www.pivotaltracker.com/story/show/59496412