Multi-server (#1906)

## Summary
Most, but not all, of the changes necessary to support multi-server throughout the app and all its features.

## Screenshots
| Light | Dark |
| ----- | ---- |
| ![Simulator Screen Shot - iPhone 13 Pro - 2021-11-26 at 21 52 24](https://user-images.githubusercontent.com/74188/143670011-9b9905ac-1b5b-4a82-b9f3-1490465c4ec5.png) | ![Simulator Screen Shot - iPhone 13 Pro - 2021-11-26 at 21 52 26](https://user-images.githubusercontent.com/74188/143670012-0080230a-8f68-4f34-9691-db9f5e825a83.png) |
| ![Simulator Screen Shot - iPhone 13 Pro - 2021-11-26 at 21 52 30](https://user-images.githubusercontent.com/74188/143670015-ceeac558-e039-4639-a186-b5001ab418b8.png) | ![Simulator Screen Shot - iPhone 13 Pro - 2021-11-26 at 21 52 29](https://user-images.githubusercontent.com/74188/143670016-d72bb69d-83f5-4197-a742-59d208467258.png) |
| ![Simulator Screen Shot - iPhone 13 Pro - 2021-11-26 at 21 52 47](https://user-images.githubusercontent.com/74188/143670021-6c90c40f-c2f1-4a33-aad9-da6626e99d9d.png) | ![Simulator Screen Shot - iPhone 13 Pro - 2021-11-26 at 21 52 45](https://user-images.githubusercontent.com/74188/143670024-e99de69d-61d8-4e12-be73-a172242806a0.png) |
| ![Simulator Screen Shot - iPhone 13 Pro - 2021-11-26 at 21 53 05](https://user-images.githubusercontent.com/74188/143670033-1a41ac7e-d4d1-458b-974e-2efdaf8e2288.png) | ![Simulator Screen Shot - iPhone 13 Pro - 2021-11-26 at 21 53 03](https://user-images.githubusercontent.com/74188/143670049-baf4db64-64db-4bfb-88cf-4930f9e5661b.png) |
| ![Simulator Screen Shot - iPhone 13 Pro - 2021-11-26 at 21 53 21](https://user-images.githubusercontent.com/74188/143670053-7ec794f1-857c-4ef6-a92a-5318e90ac6b6.png) | ![Simulator Screen Shot - iPhone 13 Pro - 2021-11-26 at 21 53 19](https://user-images.githubusercontent.com/74188/143670056-a6a5207c-3bba-49fc-b5c6-fc6fa8141f9c.png) |

## Any other notes
- Encapsulates all connectivity, token & server-specific knowledge in a Server model object which gets passed around.
- Updates various places throughout the app to know about and use Server rather than accessing said information through non-server-specific methods.
- Visually requests/notes server in places where it's ambiguous. For example, the Open Page widget will gain a subtitle if multiple servers are set up.
- Allows switching which server is shown in the WebViews. Note that this doesn't take into account multi-window support on iPad/macOS yet.

Most things will migrate successfully however adding an additional server causes things like Shortcuts to start erroring requiring you specify which to use in the particular Shortcut.

Future work necessary:
- Model objects currently clobber each other if their identifiers match. For example, both servers having a zone named `home` means one of them wins the fight for which is known to the app.
- Being remotely logged out on any account causes the app to require onboarding again, when instead it should only do that if the last known server is logged out.

Sources/Extensions/NotificationService/NotificationService.swift

@@ -9,8 +9,13 @@ final class NotificationService: UNNotificationServiceExtension {
     ) {
         Current.Log.info("didReceive \(request), user info \(request.content.userInfo)")
 
-        Current.api.then(on: nil) { api in
-            Current.notificationAttachmentManager.content(from: request.content, api: api)
+        guard let server = Current.servers.server(for: request.content) else {
+            contentHandler(request.content)
+            return
+        }
+
+        firstly {
+            Current.notificationAttachmentManager.content(from: request.content, api: Current.api(for: server))
         }.recover { error in
             Current.Log.error("failed to get content, giving default: \(error)")
             return .value(request.content)