Port advertising and article-contents modules from "Beyond the Paywall".

[rhelmer]

Fixes #48.

This depends on:
mozilla-rally/web-science#301
mozilla-rally/web-science#302

Review expectations: just code review is OK, I've tested this in dev mode and that's easy to do if you'd like but not required.

[rhelmer]

I've tested this on Firefox (which lives on a separate firefox-mv2-support branch for now) and Chrome (which is built from main), and seems to be working as expected.

The easiest way to test is to take a destinationDomains item from domains.ts and visit it in dev mode. You should see the following pings:

• one study-enrollment at install time
• userJourney every time a page visit ends (tab is closed or URL is navigated away from)
• articleContents for any domain in destinationDomains that @mozilla/readability works on
• advertisement for any domain in destinationDomains that the page-ads.content.ts script determines is an ad

The options page on the extension can be opened by clicking on the icon, or (since this behavior will change in #104) it can be reached like this in Chrome:

Or like this in Firefox (where it's called "Preferences" at least on macOS):

[aaga]

Approving so that you can merge today in case I'm offline. Nothing major