Git and history improvements over PR #3839

[realVinayak]

Previous branch and PR (#3839) had considerable branch management issues with celery worker changes. This is a testing ready version of my changes, just without the celery worker.

EDIT: The branch management issues were caused by me prematurely merging the changes from celery worker, and then working on top of them

All list items from here are valid #3839 (comment)

EDIT: Merged v7.9-dev into this

Solves #133

[realVinayak]

TODO: Need to carry over Caroline's commits to this branch

[realVinayak]

Testing instructions:

For Issue #3801

I'll recommend running the below steps for at least some of Deaccession, Disposal, Loan, Accession, Gift, Borrow tables. Also test on both v7.9-dev and this branch. v7.9-dev shouldn't be able to merge these, and this branch should.

Later on, testers can go wild and merge anything into anything - and as long as it is not an actual business rule exception, it should work.

Steps:

1. Make an interaction via the interactions form. I'll call it accession 001 for these steps. Make two new accession agents. I'll recommend creating two fresh new agents (but you can select a previous agent too). You can also make more than two accession agents. For this example, two new agents are agent 001 and agent 002. Important that both of these agents should be assigned same role in the accession.
2. Make another accession, and select agent 002 and assign any role to this agent.
3. Go into query builder, select agents agent 001 and agent 002.
4. Merge agent 001 into agent 002. The order is important to recreate issue.
5. This should fail in v7.9-dev, with an error saying found multiple records violating uniqueness constraints. In this branch, it should work without any errors.

[realVinayak]

To test lower order is taken in collector and determiners in case of deletion. v7.9 should take the most recently created one.

1. Go to collectingevent form. Add a collector. Make a new agent for this (just for testing - can also be tested with existing ones). Calling this agent agent 001. In the remarks field, put some dummy value for this collector (this is to distinguish after merge).
   
2. Save collecting event form. I'll recommend waiting a few seconds before going to next step.
3. Make another collector (with existing agent or new). Call this agent 002. Put some dummy value in remarks. Save the form.
4. Merge agent 001 into agent 002.
5. Go to collecting event form again. In the remarks, check the following:
   v7.9-dev should take the one most recently one (look at dummy value you put in remarks to confirm). This branch should take it by order (so the first one you created)

Try doing the exact same steps for determiners too.