Display a list of needs from the Need API
Bump gds-api-adapters to 7.12.0

This includes a new method on the NeedApi object to request the
list of needs
Add a unit test for the NeedHelper
Fix a couple of tests

- The functional test broke because it was trying to render the
template with our dummy data. To fix this, I've stubbed the render
action.

- The integration tests have been updated to stub the call to the
need api when we visit '/needs', and with the updated button text
for 'Add a new need'.
Merge pull request #6 from alphagov/list-needs

List needs from the Need API