This implements most parts of Pocket suggestions. They don't need any special UI
or a dynamic result type because they're only shown as the usual best match rows
or non-best match rows.
Still to do:
* Implement the "Show less frequently" behavior once we decide what the keywords
will be and how that will work.
* Implement the bottom "Mozilla Pocket" text inside the suggestion row once it's
finalized. We can use the same technique we use to show the "Sponsored" bottom
text for adM suggestions.
Other changes this makes:
* Replace the `type=bestmatch` attribute with an `bestmatch` attribute. That
lets best-match rows have a `type` too, in this case `"pocket"` (actually
either `"rs_pocket"` or `"merino_pocket"`, since I'm using the telemetry
result type).
* Improve how UrlbarProviderQuickSuggest delegates to individual features when
getting result commands, view updates, handling commands, etc., so that we
don't need to add new `case` statements for each new type of suggestion.
Differential Revision: https://phabricator.services.mozilla.com/D180059
3fd234d2a0