Add documentation comments where they're missing at the moment

[ArtOfCode-]

Our code is woefully lacking in documentation comments across the board. This makes it more difficult for new contributors as they find their way around the project - we should improve on this.

Scope initially should be started with adding comments to classes and methods that are used by other parts of our code. That means utility classes, helper methods, internal APIs etc. It excludes procedural-only code like controller actions, simple JS event handlers, etc. IOW, if it's a method defined in our code and called by our code, it should probably have a comment.