diffexp limit UI and configuration

[bkmartinjr]

Expose the differential expression cell count limit via UI, and make it configurable on the backend.

Fixes #1328

[codecov-io]

Codecov Report

Merging #1336 into master will increase coverage by 0.02%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master    #1336      +/-   ##
==========================================
+ Coverage   62.42%   62.45%   +0.02%     
==========================================
  Files          67       67              
  Lines        5198     5202       +4     
  Branches      377      377              
==========================================
+ Hits         3245     3249       +4     
  Misses       1861     1861              
  Partials       92       92              

Flag	Coverage Δ
#backend	52.73% <87.50%> (+0.06%)	⬆️
#frontend	75.26% <ø> (ø)
#javascript	75.26% <ø> (ø)
#python	52.73% <87.50%> (+0.06%)	⬆️
#smokeTestAnnotations	100.00% <ø> (ø)
#unitTest	62.45% <87.50%> (+0.02%)	⬆️

Impacted Files	Coverage Δ
server/common/default_config.py	100.00% <ø> (ø)
server/common/app_config.py	66.46% <87.50%> (+0.40%)	⬆️

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[bkmartinjr]

I have reviewed the front-end elements, which are courtesy of @colinmegill (thank you!)

[bmccandless]

LGTM