Improve attribution and appearance of credits tab

fixes #2012

The Credits page on production has the following limitations:

Authors who contributed to casebooks further back in the ancestry tree get limited credit.
Casebook TOCs are expanded in full depth, making for a cluttered display that does not add information.
Oftentimes the immediate parent of a casebook (the one that was cloned from) is a non-public draft, leading to links that 404 or return permission denied.
The labeling can be confusing (#1814).
Depending on the user's context, the Credits tab may apply to the whole casebook or just the last-viewed section, but this distinction is opaque.

In addition, the Credits view has some complex business logic but no tests.

This PR:

More clearly assigns credit to earlier authors.
Clarifies and links the original document sources from which those contributions were drawn.
Limits the the information to only the most relevant resource pages.
Always apply at the Casebook level.
Better handle cases where previous revisions were drafts that were subsequently published, or were never published drafts.
Adds a test suite.

I'm sure there are still some edge cases here but I think this smooths over some of the most common flaws in the status quo.

Before

Casebooks with lots of contributors generated hugely long lists, and obscured individual contributions. Many of these links 404 or lead to login pages because they refer to now-private drafts.

After

This shows the immediate parent casebook, plus an additional section below for earlier contributors that includes the top-level casebook or sections that were cloned:

If we explore the n-1 casebook, the immediate parent, now the two earlier contributors are foregrounded, and the specific resources are listed:

Codecov Report

Merging #2013 (87db581) into develop (50e51fc) will decrease coverage by 0.18%. The diff coverage is 95.58%.

@@             Coverage Diff             @@
##           develop    #2013      +/-   ##
===========================================
- Coverage    76.82%   76.65%   -0.18%     
===========================================
  Files           62       64       +2     
  Lines         6927     7026      +99     
===========================================
+ Hits          5322     5386      +64     
- Misses        1605     1640      +35

Impacted Files	Coverage Δ
web/main/models.py	`82.76% <ø> (+8.82%)`	:arrow_up:
web/main/test/test_publishing.py	`100.00% <ø> (ø)`
web/main/urls.py	`100.00% <ø> (ø)`
web/main/views.py	`71.88% <86.36%> (-0.17%)`	:arrow_down:
web/main/test/test_credits.py	`100.00% <100.00%> (ø)`
web/main/utils.py	`76.43% <100.00%> (ø)`

... and 1 file with indirect coverage changes

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harvard-lil / h2o