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

Up to you, but you may want to consider adding the specific commit to pull down from here.  Fine to leave as is too, to not worry about it... but, just need to be careful about breaking UK.