Investigate consolidating/simplyfing code

[denschub]

From a Chat with Tom on 2021-09-27:

i'm starting to think we should consolidate all of the code. there have been at least two cases where UA overrides needed a content script now anyway, so the code is starting to bleed together enough to consider it
we could just make everything be a type of shim, get rid of the others, and rename the shims "interventions"

There wasn't a tracking issue for that yet, so let's have this one. There are multiple ideas on how we could approach this, but one idea would be to completely remove the distinction between the current "types", and only have something like a setup() and shutdown() function for each intervention where we could do arbitrary stuff - helped by some functions for frequently used tasks.

Since there isn't too much code duplication for each intervention type at the moment, we have to be careful about not making things more complicated then they currently are... :) but it should be possible to have a couple of wins.

Let's use this issue to keep track of ideas