Add function to delete stale tokens

Summary:
- Delete old access tokens when creating a new token
- Delete old refresh tokens

Test Plan:
- Login 'LOGIN/REGISTER'
- Logout 'WORKSPACE' -> 'Logout'
- Look at the `oauth_access_tokens` to see the old access token deleted and only the new ones get added

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

Subscribers: srutto, lkamau

Maniphest Tasks: T81

Differential Revision: https://phabricator.ushahidi.com/D90