Drop unique key for {google_api_communicator_users.mail} so multiple Drupal user accounts can use the same Google account

Problem/Motivation

{google_api_communicator_users.mail} is currently marked as a unique key in the database. This means each Google account can only be linked to a single Drupal account. However, for testing purposes it would be nice to have multiple Drupal accounts use the same Google account.

Proposed resolution

Drop unique key for {google_api_communicator_users.mail} so multiple Drupal user accounts can use the same Google account.

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

None.

API changes

None.

Data model changes

Multiple Drupal user accounts can use the same Google account.