Cancel temporary user accounts at the affiliate sites.

#### What's this PR do?
- Cancels temporary users accounts on DoSomething UK

#### Any background context you want to provide?
- There are more than on and a half million users unrelated to DoSomething UK
- To handle this I provided a module update that removes these users in batches of 50
- Criterias I used to determin wheter a user is to delete:
  - User registration source is not `dosomething_uk`
  - User is not an `editor` or `administrator`
- I tested this on my local setup — works perfect

#### How should this be manually tested?
- Enable `dosomething_uk` module
- Patch your source of this PR
- Run Drupal update

#### What are the relevant tickets?
Closes [#DS-411](https://jira.dosomething.org/browse/DS-411).

Can you clarify this: ` But we can use registration date field with hardcoded release date instead. ` ?

For context, the `user_registration_source` is a field we added very recently.  We used it to track Users we import from an external site, which started around the summer I think.  For the US site, the majority of users will not have a value in this field.