Allow users to update their GPG keys.
Problem to solve
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.
User experience goal
The user should be able to update a key currently listed in https://gitlab.com/-/profile/gpg_keys to perform the above actions.
Proposal
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