Defensively add HTTP for local authority contact URLs

Councils have submitted contact URLs without a protocol, which breaks the contact link on gov.uk
https://www.pivotaltracker.com/story/show/73065224
https://govuk.zendesk.com/agent/#/tickets/718999

One of the examples is South Somerset: its current live data has been used for the test for this change. 
Would it be more robust to use `URI` and checking the `scheme`? Just in case there are any other errors.