Budget Tool: Add a private note field to payment requests

[naokomc]

What we need:

We need a new status "Needs Follow-up" & private notes section for payment requests that are under review.

• [x] "Needs Follow-up" status

When reviewing payment requests, WordCamp Central sometimes needs to put them on hold while waiting for more information or a second opinion. Currently, requests in this situation stay as "Pending Approval." The problem with this is that it's the same status as the requests that have never been reviewed. This sometimes leads to unnecessary overlapped reviews.

This is a different status from "Incomplete," which means the request itself needs some rework on the event organizer's end.

• [ ] Private notes

There is a note section that is visible to organizers. We sometimes need private notes for reasons why the request may not be approved, so deputies can understand the current situation and next steps.

Mockup

[renintw]

Reopen since there are some issues in the PR for adding the private notes. See https://github.com/WordPress/wordcamp.org/pull/1321#issue-2306512342

The request description in this issue was updated.