Generate time-series reports for casebooks and professors

[lizadaly]

This generates three new CSV-only reports that compute casebook and professor counts of three types over a period of the last 10 years:

• A count of how many casebooks were published each year by verified professors [#1978]
• A cumulative running total of how many verified professors have published casebooks by each year [#1979]
• For each verified professor, a count by year of their published casebooks.

The last item was not requested; I just misread one of the tickets. But perhaps it'll be useful!

I did some spot-checking of the counts and compared them across each other and against counts from the search page, and I think this is broadly correct. It's impossible to know when a casebook was not in the published state historically. As mentioned in the ticket, only 16 casebooks by verified professors have since been unpublished, so hopefully the absence of this information is not material. Also prior to 2021 we have only guesstimate dates for first-published dates—in reality some of this casebooks were likely published one year later than reported, because we only have creation date for that period.

Casebooks published by verified professors by year

year	Casebook count
2012	4
2013	9
2014	25
2015	33
2016	35
2017	16
2018	34
2019	31
2020	81
2021	53
2022	79
2023	17

This sums to 417—which is close to but doesn't exactly match the count of currently published casebooks because:

• This data is slightly stale
• It does not exclude casebooks in series
• It does not exclude casebooks which were published but have been unpublished

Cumulative professors with published casebooks

Year	Casebooks by professors
2012	3
2013	11
2014	35
2015	56
2016	77
2017	90
2018	106
2019	128
2020	173
2021	205
2022	259
2023	271

• Does not drop out unpublished casebooks because tracking that by date is unreliable.

[codecov-commenter]

Codecov Report

Merging #1998 (bc49aa1) into develop (4040f7b) will decrease coverage by 0.14%. The diff coverage is 60.46%.

@@             Coverage Diff             @@
##           develop    #1998      +/-   ##
===========================================
- Coverage    76.93%   76.79%   -0.14%     
===========================================
  Files           62       62              
  Lines         6871     6904      +33     
===========================================
+ Hits          5286     5302      +16     
- Misses        1585     1602      +17     

Impacted Files	Coverage Δ
web/main/urls.py	100.00% <ø> (ø)
web/reporting/models.py	97.61% <ø> (ø)
web/reporting/views.py	53.19% <52.77%> (-11.10%)	:arrow_down:
web/reporting/admin/usage_dashboard.py	100.00% <100.00%> (ø)
web/reporting/urls.py	100.00% <100.00%> (ø)

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[lizadaly]

Catherine will need to weigh in on whether this is what she's looking for and if the numbers seem truthy to her, but we can do that in a followup since this isn't user-facing anyway.