What does this change?

As suggested by @Asmau1234, this adds scalafmt to the project so that we don't have to concern ourselves about dealing with formatting.

Closes #3106

TODO

• Arrange a date and time to undertake this work when there are few pieces of work in flight
• Add instructions to the CONTRIBUTING.md file to encourage users to use "format on save" in their IDEs
• Adapt the editorconfig to ensure it does not interfere with scalafmt
• Add scalafmtCheckAll to build process so that the build process fails with bad formatting

How can success be measured?

We no longer need to include formatting issues in PR feedback.

Who should look at this?

Tested?

• locally by committer
• locally by Guardian reviewer
• on the Guardian's TEST environment