Restructure places API integration tests.

Structure it so that it's clear what operations the API supports.

This includes placeholders for the full dataset requests (currently in
cucumber), and the authority-bounded requests (not yet implemented)
Migrate integration tests for loading full data set

Migrate them from cucumber to regular integration tests.  This keeps
these tests together in one place... which is nice.
Add pending tests for authority-based lookup.
Extract service form fields into partial

To reduce duplication between new and edit views
Add location_match_type to Service model.

To indicate how places for a given service should be matched.
Add snac field to Place model.

The local_authority lookup type will match on this.
Move location lookup logic out of controller

This will make it easier to test and to add the lookup by local
authority functionality
Add MapitApi method to fetch snac for a postcode
Return places in district for local_authority services
Merge pull request #96 from alphagov/authority_bounded_places

Add support for authority-bounded place lookups