fix(gdpr): delete inactive accounts.

[teleyinex]

This new PR fixes a problem with the deletion of accounts.

First it simplifies the process:

• We check for users that have sent a taskrun more than X months ago.
• For those users, we send an email saying that if in the coming month they do not contribute again, their accounts will be deleted.
• We then run another job that will check for users that have been notified one month ago or more, if they exist, those accounts are deleted.
• As soon as an account submits a taskrun the notified_at value changes to null, disabling the deletion.

[coveralls]

Coverage increased (+0.002%) to 97.93% when pulling cdb22746e04f648410342f671c722df72a2105ea on delete-old-users into 27365573678c2e6d14ee042fb8f06e7c2807486d on master.