Audit existing tests, get back to green, and add more tests

### Expected behavior
* [ ] Our test suite should be green.
* [ ] After the test suite is green, we should lock the `master` branch and require all new contributions through green PRs.
* [ ] Test coverage should cover a larger percentage of the codebase.
* [ ] Tests should only fail for functional errors; not frontend/design changes.

### Actual behavior
* Most tests are commented out, aren't quality tests, and/or extremely old and failing.
* New tests haven't been written in many years.

### Steps to reproduce the behavior
Tests can be run with the command `rake test`:
```
drusepth@perseverance:~/Code/indentlabs/notebook$ rake test
RAILS_GROUPS is unset; defaulting to web,worker
Run options: --seed 63701

# Running:
[...failures...]
..................................................................................................................................

Finished in 37.916894s, 5.3011 runs/s, 17.7757 assertions/s.
201 runs, 674 assertions, 0 failures, 2 errors, 0 skips

Start by running `rake test` and reviewing the reported failures, errors, and existing test coverage. Done means the suite is green, coverage is increased, and tests fail only for functional errors rather than frontend or design changes.

Written by the indexing model from the issue text.