Decide if user can change their email to any arbitrary value and change interface accordingly

Currently the interface will allow a user to change their account email to be their authrocket login email, but will NOT allow any other kind of email changes (see https://github.com/biocore/microsetta-private-api/pull/170#discussion_r437034656 ). If we want to allow arbitrary email changes, we should expand the account_details update functionality. If we DON'T want to allow arbitrary email changes, we should provide some sort of UI info to the user about the fact that they can't do that. Related issues are biocore/microsetta-interface#47 and biocore/microsetta-interface#48.