v2 API should include source-level properties for unread and file attachment status

[legoktm]

Description

During performance optimization of freedomofpress/securedrop-client#2559, we realized it would be valuable to have source-level properties for:

• is_read - are all the items in the source's collection marked as read
• has_attachment - is any of the items in the source's collection a file

The implementation will look something like #7552 (comment) (and this issue supersedes that one).

[zenmonkeykstop]

Let's scope this for 2.13.0 if we can. If so, we can likely use the model change with the v1 api as well and get a performance boost in the current client with little effort, which would make existing users' experience a lot better in the interval before the new app drops.