Add Source model
Add, edit, update and delete sources for a need
Combine sources into a single table

This moves Directgov links, existing services and
justifications into the Sources table instead.
Their kinds are carried across too.

This migration is reversible, with the exception
of the Directgov link titles as Directgov can no
longer be reached to retrieve these.
Remove Directgov links, existing services and justifications

Also adds sources to the need page.
Add model spec for Source
Add feature specs for adding a source
Add extra css class to sources list
Add feature specs for editing and deleting sources
Remove DirectgovLink spec
Remove directgov links and existing services from the needs CSV export
Remove old source fields from need edit form
Drop the old source tables
Change 'directgov' to 'legacy_service'
Merge pull request #15 from alphagov/need-sources

Combine Directgov links, existing services and justifications into sources