Use consistent style for trailing commas

I'm a fan of leaving trailing commas in multi-line array/hash literals and method calls since it leads to cleaner diffs and less errors with missing commas, but the current style in the codebase is inconsistent and not enforced.

You can find the current instances of trailing commas with something like ag ',\n\s*[})]', there are a couple hundred of them.

Rubocop offers Style/TrailingCommaInArguments (method calls) and Style/TrailingCommaInLiteral (arrays/hashes) to handle these.