Migration plan from old notifications settings to the new ones

We're changing our notification settings. Users will not have the same selections available to them. We need to provide a way to migrate (or manage) the move from the old system to the new system.

```[tasklist]
### Tasks
- [ ] Add a banner at the top of the settings page for existing users explaining that we've updated the screen but their previous choices remain the same
- [ ] Add a "Custom" choice to the 'Default' section in the new global screen if the users previous selection does not match the new options
- [ ] "Custom" should also be visible (and selected) if user's have set their notification settings on a different client and there's no obvious mapping back to our view here
- [ ] Once unselected, "Custom" should not be available to choose.
```