UK SSO: adapt login/signup to common flow provided in external_auth.

#### What's this PR do?
- Adapts Dosomething UK module: use login/signup provided in [`external_auth`](/dosomething/drupal-external-auth)

#### How should this be manually tested?
- Enable and setup Dosomething UK module
- Run `ds test uk`  
![image](https://cloud.githubusercontent.com/assets/672669/4666162/f8c868ae-5550-11e4-8beb-254f26963bd1.png)

#### What are the relevant tickets?
- Part of [#DS-379](https://jira.dosomething.org/browse/DS-379): Move functionality common to UK and Canada to the new nodule.
- dosomething/drupal-external-auth#2

Picky, but should read as `Implements hook_external_auth_method()` right?