Enable TypeScript strict mode by ignoring lots of errors

[seancolsen]

Fixes #619

In this PR

• I turned on TypeScript strict mode.
• About 1/2 of the errors were easy to fix, so I fixed them.
• I suppressed the remaining errors, linking to Fix all ignored TypeScript errors #1055 for more info. I'll flesh out that ticket after we get this PR merged.
• I also upgraded TypeScript because the improved control flow analysis in 4.4 helped to eliminate some of the errors in display.ts. I had to upgrade ESLint after upgrading TypeScript.

Notes

• Unfortunately it seems there is probably no way to suppress TypeScript errors in a Svelte template. So I had to lift some code out of templates and into the script just for the purpose of suppressing the TS error.
• In addition to TypeScript showing more errors in strict mode, ESLint also shows a (slightly) more errors now because it knows more about the variables. As such, some of the changes I made were necessary to appease ESLint specifically, not TypeScript.

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Checklist

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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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[seancolsen]

@pavish @kgodey Since we have a nice lull in the PR queue right now I think it'd be great if we could get this PR merged before any new front end PRs, otherwise we might end up chasing conflicts for a while like we did with #826. I realize we have a couple full-stack PRs that will contain font-end changes, so I'm open to merging this PR either before or after those.

[kgodey]

@seancolsen I'll leave it to you and @pavish to determine merge order; I don't know enough to have an opinion.

[pavish]

@seancolsen This PR looks good to me!

I think it would be best if we could merge #1041 and #1047 first. Both these PRs seem ready on the backend, and we could add frontend changes there and merge them and handle any new ts error in this PR.

This PR could then be merged right away.

[pavish]

@seancolsen It looks like #1047 is not ready yet. I'm approving and merging this PR first.

[pavish]

I don't like the name of this const because it goes against everything we've established on the frontend. I believe this particular scenario warrants a discussion and some documentation updates.

[codecov-commenter]

Codecov Report

Merging #1056 (bed3144) into master (d010e11) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1056   +/-   ##
=======================================
  Coverage   92.53%   92.53%           
=======================================
  Files         108      108           
  Lines        3830     3830           
=======================================
  Hits         3544     3544           
  Misses        286      286           

Flag	Coverage Δ
pytest-backend	92.53% <ø> (ø)

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