Bug 1861248 - Overhaul ASRouter admin and remove its dependency on newtab. r=omc-reviewers,emcminn

For starters, replace all newtab styles and functions. Since we're
removing these styles, I conformed the asrouter admin to the reusable
components team's design tokens. So, it now uses global system page
styles. This is a pretty general overhaul since there are so many styles
to replace. In addition to the style changes, I've added a new Filters
UI and moved the groups table to the General tab. This allows us to
remove the Message Groups tab since that functionality is now rolled
into the Filters UI. The same with the Private Browsing tab: when you
hit Show on a pb_newtab message, it will open a PB window and override
the message. And you can filter by template now, so you can view only PB
messages on the General tab. I also fixed spellchecking. Instead of
spellchecking, which only works for natural languages, we just validate
that the text is valid JSON. If it's not valid, we show a red border on
the textarea. That way messages won't just mysteriously fail to show. I
also moved a few elements around to conserve space. Now, everything
should be able to fit on the screen of a default window size of 1500px.
Finally, I removed some old cruft that was left over from when the admin
interface was part of the newtab page.

Differential Revision: https://phabricator.services.mozilla.com/D213865

browser/components/asrouter/modules/PanelTestProvider.sys.mjs

@@ -341,7 +341,7 @@ const MESSAGES = () => [
         },
       },
     },
-    groups: ["panel-test-provider"],
+    groups: ["panel-test-provider", "pbNewtab"],
     targeting: "region != 'CN' && !hasActiveEnterprisePolicies",
     frequency: { lifetime: 3 },
   },