[Very meta] Add code review guidelines

[pcraig3]

@brdunfield, sorry to make you the first-ever person to be held to this Very High standard but someone's gotta be.

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I went over a bunch of different sources for this. I tried to split the difference and come up with something that is useful for reviewers to read, both in terms of the tone of reviews and in the more technical terms of what we should look for.

Here are my sources:

• https://github.com/alphagov/styleguides/blob/master/pull-requests.md
• https://github.com/cds-snc/.github/blob/master/CODE_OF_CONDUCT.md
• https://github.com/alphagov/styleguides/blob/master/git.md
• https://github.com/thoughtbot/guides/tree/master/code-review
• https://www.evoketechnologies.com/blog/code-review-checklist-perform-effective-code-reviews/
• https://www.codementor.io/blog/code-review-checklist-76q7ovkaqj
• https://www.michaelagreiler.com/code-review-checklist/

[sonarcloud[bot]]

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