Issue 7110 - Only show icon popup notification for pages that match URL filters

This MR for #197 (closed) contains the following changes:

• Added new, optional parameter to "notifications.get" message to retrieve only notifications that are either global or target the given URL
  • Updated docs
• Moved function to retrieve currently active tab to js/popup.utils.js to be able to reuse it

Note that, due to the little time we have to land this change this year still:

• I've decided to (mostly) duplicate the logic for matching URLs against URL filters because that logic is currently not exposed by Core. However, I'm going to create a follow-up Core ticket for it so that we can clean that up without running the risk of delaying this change.
• The issue has yet to go through the various phases to get approval from all stakeholders
• I've based it on release-2018-5 for now because it's not clear yet, what branch this should land in (presjmably release-2018-5.-8, in case release-2018-5.-9 is supposed to land beforehand)