Add ability to tombstone user accounts

[ml-evs]

I wrote a long description but it was swallowed by SNCB wifi, basically need a mechanism to be able to prevent people from logging in/writing, whilst still retaining their data. Perhaps important for #468

[ml-evs]

Tombstoning should also have an option for full anonymisation (I think by law), I'm not sure how this interacts with e.g., wanting to keep an email address for contact regarding samples, and the logic here becomes very convoluted (i.e., if a user tombstones their account and thus can no longer login, how do they request for their data to be anonymized).