Remove Canvas LMS features from server codebase

[patcon]

Blocked on: https://github.com/compdemocracy/polis/pull/1235 (After that's merged, I will use the GitHub UI to switch this PR's base branch from micahstubbs/960/typescript to dev) Related: https://github.com/compdemocracy/polis/issues/263

(Ignore all but the last 2 commits, as these first ones are in other PRs or already in dev)

This PR cautiously removes all references to Canvas LMS, including:

• a commit for removing code paths from each of server
• a commit to add a SQL migration, removing 5 canvas-related tables

Of note:

• stepping through relevant commits might be the easiest way to review right now
• this removes ~1700 lines of code
• I removed code related to "LTI", "canvas", "assignments", "grades", and "teachers"

Open Questions

None!

[patcon]

Re: e2e test failures.

Seeing tons of issue with POST /api/v3/auth/new in logs

The code changes do affect /api/v3/auth/new and /api/v3/auth/login, but they passed before this PR was rebased, so I'm thinking it may be something else.

I suspect it has to do with the changes to the server image in #961 where npm run build:watch is now part of the docker image CMD. This is probably taking too long to start/rebuild during some important test setup (I suspect of the test users), and so the rest of the tests (that requires these users) are failing.

Relevant logs: https://github.com/compdemocracy/polis/runs/4514842209?check_suite_focus=true#step:15:14

Fixing will likely involve reverting the changes to the dockerfiles, as @metasoarous mentioned elsewhere :)

[ballPointPenguin]

continued in / closed in favor of https://github.com/compdemocracy/polis/pull/1620