Replace close and anonymise with independent account closure actions

[garethrees]

Requires #7611.

Instead of a single "close and anonymise" action, we should make the set of actions more composable. For example, a user may only want their account closed rather than anonymised.

Each option should document what the action does inline in the UI. Might want to fold the specifics in a <details> tag if it starts getting too long.

For close and anonymise we currently state:

This is disabled for banned users as the email address must be preserved to enforce the ban. First remove the ban text and then close and anonymise the account.

I think this is still true. We should allow the #close and #anonymise options when the user is banned, but disable #erase, as that's the one that will now remove the account email address.

When an account gets closed, we should ensure that no email gets sent to it (#4603 is likely relevant here). We'll need to be careful about this becoming a rabbit hole. If it starts getting complicated, the easiest solution might be a callback along the lines of before_action { return false if @recipient.closed? } in the mailer methods.

Note that #5052 (comment) still requires the User#close_and_anonymise method – this issue is just to replace the admin UI options.

[JenMysoc]

Next step

• Merge the PR and close this off