Run elm-format on CI

[parkerl]

Because we are being strict about formatting we should fail the build if elm-format causes diffs.

[justgage]

I would be a little afraid that this would cause people to become a little discouraged. I think suggesting to use it with a bot in a comment after they submit it could be a good thing.

[kytrinyx]

@justgage To clarify this about pull requests on GitHub to the Exercism Elm track, not about failing the tests when people are submitting exercises to the website.

[justgage]

Ah yes, that's a good idea. Sorry about that.

[kytrinyx]

No apologies necessary—you've got everyone's best interest in mind (and I agree with you: this would be frustrating and potentially quite discouraging if we did it on the website).