[cms] Allow commenting on same-domain invoices

[xaur]

Invoices often have issues like malformed data, unclear descriptions or hour amounts that need more clarification or breaking down.

It would be helpful if contractors could comment on same-domain invoices to help each other craft invoices that are easy to audit.

Same visibility rules as in #1225 may be used, i.e.

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.

Although it is quite restrictive and does not allow reviewing contractors to see each others comments. I would make it more relaxed like:

Same-domain users can see each other's comments and comment on each other's invoices, but cannot see threads started by admins.

[xaur]

Per chat discussion, allowing all domain peers to comment on an invoice opens the risk of people spamming it with too many questions and frustrating the contractor too much.

The issue of building a productive and scalable review process, and recording all important messages in CMS storage still stands, but this risk must be taken into account.

We will start with discussing issues found in invoices via chat DMs.