Allow notifications to open extension page

Background

In https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui/issues/378, one of the requirements is that clicking a specific notification should open a landing page that we're bundling with the extension. Currently, however, notifications can only link to documentation links.

What to change

Change the behaviour of notification button links so that:

1. If the link contains at least one "/" character, treat it as a URL and open it in a new tab.
2. Otherwise, pass the link to getDocLink before opening the result in a new tab.

Take care to make this behaviour consistent with the "Open all links" button as well.

Notes

• Older versions of Adblock Plus will still pass all button links to getDocLink, which will result in a link like "https://google.com" ending up as "https://adblockplus.org/redirect?link=http://google.com&lang=en". We might consider doing something for those cases server side in the redirect handler.

Hints for testers

This change was superseded by #68 (closed), so there's nothing to test here. Instead check out the testing hints for #68.