[cms] Proposal owner invoice approval

[alexlyp]

The following is the current thinking of how we'll accomplish the functionality for proposal owners to offer invoice approval when the invoice contains a line item that has a proposal token that they own.

1. Proposal owners will be able to see invoice details that only pertain to their need for approving: Line items that they control, the contractor rate and the contractor username.
2. The proposal owner will give an approval or be able to comment on the invoice.
3. If approved it will send a metadata stream update to the backend that will include a proposal-owner signature of a json payload of the line items they were able to review.
4. Administrators will not be able to fully approve invoices until proposal owners first approve their line items.
5. Proposal owners will be notified of needing to review/approve line items on invoice creation.
6. Proposal owners will be allowed to comment on invoices, but will only be able to see threads that they begin. Administrators will be able to see all comment threads.
7. Administrators will be given the ability to ping proposal owners that they have reviews waiting on an invoice.

[xaur]

Administrators will be able to see all comment threads.

Will invoice authors see all comments too?

[alexlyp]

Yes

[xaur]

Currently proposal owners cannot see unpaid billing against their proposals because of #1329 so I guess that one needs a fix first (separately or as part of #1243).