Add private notes to vendor payments and reimbursement requests

Only network managers (i.e. super deputies) can see these.

How to test?

1. Log in as a site admin (not a network or super deputy)
2. Crate reimbursement request
3. "Private notes" meta box should not be visible
4. Log out and log in as an network admin
5. Go to request
6. "Private notes" meta box should be visible
7. Leave a note, check that it is showing and you didn't receive any email notifications

Add a new "Needs follow-up" status for payments

As requested by Community Team deputies who are working with payments. Added this status for vendor payments and reimbursement requests. Also, the network payments tool takes this status into consideration when combining payments for the overdue view.

How to test?

1. Log in as a network admin
2. Open one and edit it, the staus should be visible
3. Visit wp-admin/network/admin.php?page=reimbursement-requests-dashboard and tab "Needs follow-up" should be there
4. On a WordCamp site, create a vendor payment and set due date to past date, set the status to "Needs follow-up"
5. Visit /wp-admin/network/admin.php?page=wcp-dashboard and the payment should be listed on "Overdue" tab and separate "Needs follow-up" tab should be visible

Extend notes to vendor payments

For ease and harmonisation, the regular notes meta box was extended for vendor payments. Only users with view_wordcamp_payment_details (network admins and original submitter) can add new notes, but everyone can see all notes. Because of this, the note functionality was moved to common WordCamp_Budgets class.

How to test?

1. Log in as a site admin
2. Go to vendor payments
3. "Notes" meta box should be visible
4. Log out and log in as an user who has not made the vendor payment
5. Go to payment and meta box should be visible but you are not able to add a note.

Fixes #864

Screenshots