Issue 595: Allow admins to edit a new project request's PI

[Hzaakk]

How to test:

• Open up a new project request and review the first admin checklist item.

[matthew-li]

From today's meeting:

• Make the PI selection field filterable.
• In the help text for that field, explicitly note that, if they select "New PI", they must provide first, last, email. And that if they don't, they should not fill in those fields.
• In the error message, "PI information is incomplete", also call out these required fields.