test: add Cypress and Percy tests

[nicholaschiang]

Fixes PD-11 by creating:

• [x] Cypress functional tests, and;
• [x] Percy visual tests (screenshot diffing), and;
• [x] GitHub Actions CI to run tests after every git push.

[linear[bot]]

PD-11 Add Cypress integration tests and CI

- [ ] Add Cypress integration tests for all the key flows in the app. I'm aiming for 100% code coverage using integration tests alone. - [ ] Setup GitHub Actions to run our build and integration tests after every `git push` (on both Firefox and Chrome). - [ ] Add codecov, Cypress test status, and build status badges to the `README`. ETA - 90mins

[vercel[bot]]

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hammock – ./

🔍 Inspect: https://vercel.com/readhammock/hammock/6FYkZe62zMHUgRW55VMohG2cov9h
✅ Preview: https://hammock-git-cypress-readhammock.vercel.app

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (develop@510fddc). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop     #135   +/-   ##
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  Coverage           ?   38.82%           
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  Files              ?       80           
  Lines              ?     1306           
  Branches           ?      377           
==========================================
  Hits               ?      507           
  Misses             ?      799           
  Partials           ?        0           

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