Implement file approval backend

[madwort]

as per #137

[bloodearnest]

Look good, all the right pieces in place.

Left a few comments.

However, I am not a fan of get_file_approvals design in the ticket, and deserialisation between the DAL and the BLL.

From my comment there:

I would much prefer it if business_logic.RequestFile grew an
.approvals attr, which is a list of the new
business_logic.FileApproval objects for that file. And that when
get_release_request will build up the full tree of objects via its
existing from_dict mechanism, and we can just iterate the one
object, rather than integrating two data sources.

When we call bll.get_release_request(...), we want to get back a single object representation of everything, like so:

ReleaseRequest
  .filegroups[]
     .files[]
         .reviews[]

Rather than having to call .get_release_request and also then .get_file_approvals and pass both into wherever they are used, its all in the one object tree.

I cannot think of a use case for getting the approvals alone, when we don't also need everything else, but maybe I've missed that?

I think if we do this, it should solve a few issues with this PR atm:

• removes the need for the circular FileReview.release_request reference
• makes the from_dict implementation simpler.

[madwort]

I cannot think of a use case for getting the approvals alone, when we don't also need everything else, but maybe I've missed that?

I started on this path because I wanted exactly that for the unit tests & then decided to proceed with the implementation in the original ticket. Having looked at it again, though, I think you're absolutely right so have re-worked this in the way that you suggest.

[bloodearnest]

This is nice, thanks for reworking those bits. I think its worked out as I'd hoped 🙏

Left a couple of suggestions, but happy to land this!