[cms] Proposal owner invoice approval

[alexlyp]

Closes #1225

This PR adds the ability for users that are designated as "proposal owners" to approve line items that have been billed against the owned proposal.

Now proposal owners are able to see invoice details that will show them line items of proposals that they own. They can now also comment on an invoice to communicate with the invoice creator if need be. They will be able to see any comment thread that they started, but not admin or potentially other proposal owner comments.

If the invoice looks appropriate then they can "approve" the invoice. This request will update the invoice record on the backend with a new metadata type that contains the line items payload and their signature of them. This is more or less "proof" that they have approved those specific line items.

Currently, there is no more handling of approved invoices other than just a list of proposal tokens in the invoice record. In the future we can update status or something more involved.

[xaur]

a new metadata type that contains the line items payload and their signature of them

@alexlyp to clarify

1. does it mean a signature by the proposal owner of the line items is saved on the backend?
2. only line items using a certain proposal token are signed and not all line items, correct?
3. is it possible for proposal owner and invoice author to retrieve these signatures?
   • these cryptographic attestations may be useful for disputes and investigations and it should be possible to have a copy just like with the proposal bundle on Pi (or like when you sign two copies of a document in the real world)
4. will same-domain peer reviewers see these signatures?