Allow admins to edit a new project request's PI

[matthew-li]

Per #594, users sometimes select the wrong PI for a new project request.

There's currently no way to correct a wrongly-selected PI from the website. Update the new project request detail view such that admins (but not the requester and PI) have some way to update the PI.

• As in the normal request flow, the PI may be selected from a dropdown (if already existent) or created from name + email (if not).
  • It may be simplest to have one form with both the dropdown and the fields for a new user, rather than having a multi-step form.
    • Validate that only one method is used (i.e., dropdown must be empty if new-user fields are filled in, and vice versa).
  • The same constraints as in the regular flow must still apply (e.g., PIs are limited to one allowance (for some types of allowances); on MyLRC, PIs must be LBL users, etc.).
• It may make sense to integrate this into the admin checklist step form for confirming PI eligibility.