Users should be able to remove their account

[flodolo]

Right now they need to message an admin, who needs to search for the user and remove it. That seems unnecessary complicated.

[mathjazz]

Note that there's already #2171, but I think we should close it. I can't think of a scenario in which we'd need manual intervention before removing an account.

[flodolo]

I agree that we should drop #2171. I'm honestly surprised by the amount of people signing up "by mistake".

As part of this, we should also add a note to the first onboarding email about removing the account if they didn't sign up for it.