Implemented site isolation

Added feature to isolate (lock) assigned sites: When you are in
a container with site isolation enabled, navigating to a site
outside of the assignments will open that site in a new default
container tab.

Co-authored-by: Francis McKenzie <francis.mckenzie@gmail.com>

test/features/assignment.test.js

@@ -14,7 +14,7 @@ describe("Assignment Reopen Feature", function () {
     this.webExt.destroy();
   });
 
-  describe("click the 'Always open in' checkbox in the popup", function () {
+  describe("set to 'Always open in' firefox-container-4", function () {
     beforeEach(async function () {
       // popup click to set assignment for activeTab.url
       await this.webExt.popup.helper.clickElementById("always-open-in");