Format: Trailing commas

[sebastian-peter]

My proposal is to use trailing commas. They can reduce the amount of lines changed and simplify merges with conflicting lines a lot.

This webpage gives a short introduction, and this blog post explains the benefit with merge conflicts.

spotless for Scala has an option to enforce them, no extra work required from our side: rewrite.trailingCommas.style = always

[sebastian-peter]

Created issue: #719