Extract a `Gems::BaseController`

[kaspth]

We've got a subsection of the app that expects and works off of a @gem variable. I think it'd be clearer if we made a dedicated hosting place for that than putting it in a concern.

Additionally the other controller concerns seem like premature extractions, some only being used in lone controller show actions. I think it'd be clearer if we inlined them for now.

The more complex finder is the set_target one that's used in both the gem class/instance methods controllers. It seems like they both share the use of params[:class_id] and params[:module_id] with a fallback to the general analyzer. I don't quite understand the general fallback to the general analyzer, but I kept it for now.