More style fixes

This is the second part of my crusade against non-standard code style; the first part being #476.

There are two reasons why these changes were not in #476:

1. Rubocop got updated with a lot of new checks, and this PR ensures we adhere to the guidelines set forth in that new Rubocop.

2. I got over the attachment to my personal preferences (such as TODO’s not being postfixed by a colon).

I will probably squash this PR into a single commit, since it gets quite noisy otherwise.
I'm with @ddfreyne … consistency matters more than just about anything else when it comes to code style. Sure, everyone else has dumb preferences, but it doesn't really matter. We have tools to check and even fix all of it, so just pick the most widely used standards, set up all the tools, and quit worrying about. You've probably got more important problems to solve :)