Remove UserConfigRepository from repository module

This is now superseded by UserConfigHttpRepository introduced in #285.
The approach I'm taking to do this removal is remove the code and then
make whatever changes are necesasarry to the tests.
Remove UserConfigRepository from unit tests

This removes the now redundant UserConfigRepository from the unit and
integration tests found in /tests. The behaviour tests will still be
failing.
Add tests for StagecraftService

Some of the behaviour support classes are quite complicated. This one in
particular I'm about to add another feature to and would like to have it
tested before I do that.
Allow adding routes to a StagecraftService

- Allow new routes to be added to a running StagecraftService
- Allow routes to be reset

This is going to be used to allow the admin feature tests to be migrated
to using stagecraft. To make this possible they will need to be able to
add both data-set routes and user routes to the same service.
Fix imports
Add functions to start/stop/update stagecraft mock

Add two functions to start/update and stop a StagecraftService based on
a behave context.
Use StagecraftService for users and data-sets

- Make use of new functions to start/stop/update the StagecraftService
- Use StagecraftService for serving user data in behave tests
PEP-8
Tidy up feature imports

- Move `api_common` module from steps to support as it contains no steps
- Rearrange all imports to:
  - Use relative imports where appropriate
  - Separate imports into system, 3rd party and local
  - Alphabetise imports within each group
Merge pull request #294 from alphagov/remove-mongo-user-repository

Remove mongo user repository