Fix gpg key deletion (#14561)

Fix gpg key deletion (#14561)

* Fix GPG key deletion when user is deleted

Per #14531, deleting a user account will delete the user's GPG keys
from the `gpg_key` table but not from `gpg_key_import`, which causes
an error when creating an account with the same email and attempting
to re-add the same key. This commit deletes all entries from
`gpg_key_import` that match any GPG key IDs belonging to the user.

* Format added code in models/user.go

* Create a new function for listing GPG keys and apply it

Create a new function `listGPGKeys` and replace a previous use
of `ListGPGKeys`. Thanks to @6543 for the patch.

Co-authored-by: Anton Khimich <anton.khimicha@mail.utoronto.ca>
Co-authored-by: 6543 <6543@obermui.de>