UCL A/B Testing

This pull request contains work necessary to implement #76.

## Number Display

* [x] Implement testing sessions.
* [x] Implement displaying of numbers for selected candidates.

## Tracking

* [x] Record postcode search.
* [x] Record visit composition page.
* [x] Record message send.
* [x] Implement test session token saving to message table.
* [x] Record message confirmation.

## Surveying
* [x] Implement survey.
* [x] Ensure survey is correctly linked to testing sessions.

## Tidying

* [x] Update privacy policy.