Support parent selection in breadcrumb navigation and new child creation

[willbarton]

This PR adds support for maintaining parent selection while navigating the tree in breadcrumbs and when creating a new child.

Additions

• Clicking on a model name in the breadcrumbs while having a parent selected will maintain the parent selection throughout the tree.
• Creating a new child from a page with a parent selection (either the + button on an empty parent or the "Add {Model Name}" button) will have that field in the creation form filled out with the selected parent.
• Editing a child will redirect to the parent-filtered listing page.
• Deleting a child will redirect to the parent-filtered listing page.

Changes

• Fixed the "Edit" button under the parent name. Previously it was editing the child object with the parent's primary key.
• Refactored some of the TreeIndexView parent logic into a mixin class.
• Refactored the breadcrumb logic to make use of a URL helper subclass.
• Changed treemodeladmin/index.html to extend modeladmin/index.html and removed all the blocks that aren't modified by TreeModelAdmin.

Testing

1. Standlone (thanks, @higs4281!):

   export DJANGO_SETTINGS_MODULE=treemodeladmin.tests.settings
   export PYTHONPATH='[PATH TO PROJECT]'
   django-admin.py migrate
   django-admin.py loaddata treemodeladmin_test
   django-admin.py runserver

   Then visit http://localhost:8000/admin. Author will be available under the "TreeModelAdmin Test" menu item.

2. With cfgov-refresh and docker:

   git checkout origin/treemodeladmin
   docker-compose build python
   docker-compose up

   Then visit http://localhost:8000/admin. The Regulations 3000 models will be navigable via TreeModelAdmin under the "Regulations" menu item.

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows the standards laid out in the development playbook
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Visually tested in supported browsers and devices (see checklist below 👇)
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️

[higs4281]

This works well for me and is a great improvement.

One nit I noticed was that when adding or editing an object (such as book in your demo), after saving the admin URL loses the ?author= parental reference, and I see all books.

I don't see including all the children as bad parenting per se.

[willbarton]

@higs4281 Ah yes, that is an oversight!

[willbarton]

@higs4281 alrighty, that should be fixed now too! Creating, editing, and deleting an object should redirect to the parent-filtered child listing!