Add option for users to retract their comments and likes on deleting an account

This is a split off #7848 to avoid mixing two issues.

In the account deletion confirmation popup, we should add a checkbox allowing the users to also have their comments and likes retracted. Although I don't really like this idea as it completely ruins context for everyone, we probably should allow this regardless, and in the lights of GPDR, it might actually be required by law (but nobody knows yet).

In the lights of recent discussions, I have marked this issue as blocking-release without a target release, just to make sure someone feels compelled to work on this with higher-than-average priority.