Allow users to update their GPG keys.

GPG keys needs to be updated from time to time: either subkey rotation, key expirations, inclusion of subkeys or uid revocations, etc etc.

This is not possible currently, one needs to delete the current key and re-upload it.

The user should be able to update a key currently listed in https://gitlab.com/-/profile/gpg_keys to perform the above actions.

Allow to paste in the "Add a GPG key" form a key that is already listed, allowing for its update (perhaps after double-checking with the user).

Doing so currently yields the following form error:

The form contains the following errors:
 * Fingerprint has already been taken
 * Primary keyid has already been taken