Add context menu to set cell to null

[seancolsen]

Fixes #930

Demo

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the master branch of the repository
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• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

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[pavish]

@seancolsen Looks good to me!

[pavish]

I'm not merging this since the base branch is context_menu for which I had a couple review comments. Once the base PR is merged, feel free to merge this into master.

[codecov-commenter]

Codecov Report

Merging #1215 (ab5b4ca) into master (b94bfa8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1215   +/-   ##
=======================================
  Coverage   93.41%   93.41%           
=======================================
  Files         113      113           
  Lines        4341     4341           
=======================================
  Hits         4055     4055           
  Misses        286      286           

Flag	Coverage Δ
pytest-backend	93.41% <ø> (ø)

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