add email to update_credentials

[lanodan]

Related to: https://git.pleroma.social/pleroma/pleroma/merge_requests/1580#note_37605

The API allows to create an account, but it doesn't allow to change the email later on (might be true for other data too, I haven't checked).
Pleroma would want to add it to the update_credentials endpoint, could it be done on mastodon too to avoid useless differencies?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[progval]

This issue is still relevant