Allow module optionally to add Siteimprove analysis code to site

Here's my attempt at a reroll for the 2.0.1 version of the module. No major changes - mainly had to move stuff around.

There is a module for this: https://www.drupal.org/project/siteimprove_analytics →

I'll close this issue for now. If it will be decided to add that functionality to this module (e.g. as a sub module), that will be handled then.