Add option to delete personal data

[lukavdplas]

Users should have an option to have their personal data purged from the database.

Concretely, this means:

• deleting your search history
• deleting your downloads
• deleting your account (including search history and downloads)

[JeltevanBoheemen]

• search history
• downloads
• account

[lukavdplas]

Re. downloads, I remember why said this may not be necessary.

In #947, I suggest that downloads should have an "expiration date". It makes sense to me that we just delete downloads after, say, 90 days. You could still add an option to delete all your saved downloads right now, but it's less necessary in that case.

[lukavdplas]

We recently added deleting accounts to lettercraft: CentreForDigitalHumanities/lettercraft#157

Since lettercraft's user system is based on I-analyzer, it should be easy enough to take this PR and adapt it back to I-analyzer.