[ui] User deletion

According to legal advice, the BEAT platform must honour the "right to be forgotten" as is now being required by applications hosted on EU countries.

A user may be requested to be deleted, together with all (private) contributions. For public contributions, we have the right to keep them (fortunately). Here are the actions to be taken when a user is removed:

• Public components are kept, but ownership is transferred to an anonymous user.
• Private components are removed
• The user account details (e-mail and such) is removed
• Attestations are kept if they are published, Locked attestations are deleted
• Users that have forked something from the said user are notified (?)