Implement "copying" of cohorts/variable sets/constructs/whole projects between different users

[Viqsi]

This is kind of our answer to the collaboration problem posed by #126 - instead of directly sharing projects and constructs (and thus having to manage all sorts of state synchronization issues), we instead have users copy them back and forth as needed - "send your work" and so forth. It's also very strongly dependent on #162.

There are some concerns w/r/t who gets to make the call about overwrites and prompting and the like. The rough workflow concept I currently have in mind is as follows:

• User A has project Shiny that contains constructs Foo, Bar, Baz, and Thing, and cohort Folks. Baz is dependent on Foo, Thing is dependent on Bar and Baz (and so is also dependent on Foo via Bar), and Folks is based off of Thing.
• A wants to be able to share with user B. So A puts in a request to copy Shiny over to B. The request is put into the database, and B is prompted (when their client notices) to accept the copy.
• B opts to change around Thing a little bit and re-create Folks. They share this with A by sending copies for Thing and their new Folks back to A. A is prompted as to whether or not to accept these, and is warned that their own copies will be overridden if they do. (They're tracked exclusively by name; if B recreated Folks under the name BFolks, no overwrite would take place if it was copied to A and A accepted.)

[Viqsi]

This one may end up getting pushed to Public Launch or even Aspirational. We've got clarity on whether or not we want some copying, but not the timing or extent.

[Viqsi]

One possible concern is that depending on how we implement this, user B could end up with multiple copies of the same cohort tree (for example), because there's nothing tying A's "Cohort X" to B's "Cohort X" and we can't know for sure if they're changed or not.

I mean, I suppose we could do a by-name comparison for every one but that makes me want to flinch. It's straightforward to do that for cohorts (compare contents of cohortInds) and variable sets (compare variable lists), but constructs? Hoo boy.

[Viqsi]

Also, since it's between users and users may not be on simultaneously, there needs to be a concept of "pending copy requests" stored in the database. That's a much more complicated enterprise w/r/t the UX.