* Adds a `remoteType` attribute to the config when registering a sidebar to opt-in to using a remote content process when displaying that sidebar panel.
* Moves loading URLs into a `SidebarController._loadUrl` method, which can be passed a remoteType if the sidebar contents should be displayed in a remote content process. If needed it will replace the sidebar browser element with a remote or non-remote browser to match the remoteType passed.
* Adds browser load listeners methods, a `SidebarController._hasLoaded` promise for non-remote loading and a `SidebarController._hasProgressStopped` promise if the browser is remote.
* Adds a setter for `SidebarController.browser`.
* Adds a `shopping-sidebar` message manager group to avoid actor conflict.
Differential Revision: https://phabricator.services.mozilla.com/D223961
5c68a50632