Introduce Swiftlint

[bfollington]

Trying out a basic set of rules for catching mistakes like line length etc. everything is configured as a warning rather than an error right now so the build will never fail for lint reasons. I started with all the rules on and removed anything that was obviously in conflict with our existing style / rules.

We have ~500 lint warnings in the codebase atm and it would be good to fix them because github actions can only display a maximum of 10 linting errors per run (yeah, I know). So lint errors introduced in a PR won't actually show up in the UI until we clear the backlog of underlying lint issues.

The github action I used (made by the author of Swiftlint) purports to be able to scope the linting to only files changed in the PR but I get errors whenever I try to configure it as per their example.

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Swiftlint can fix basic issues by running swiftlint --fix but we need to pick our moment carefully to avoid conflicts with in-flight work.

You can also choose to integrate swiftlint into Xcode as a build action (see this draft PR #573). If we gain enough confidence in rules I would be tempted to add a pre-commit hook that autofixes.

[gordonbrander]

LGTM.

Let's pick a time to merge this when it won't cause a bunch of disruption.