Initial implementation of AccountSettings. (#6275)

* Make the UserPreference macro also generate an accessor for the key name.

* Initial implementation of AccountSettings.

- Add an AccountUserDefaults storage with a userID prefix on the key.
- Migrate the 2 values reset by AppSettings.resetSessionSpecificSettings to AccountSettings.

* Rename UserSettings.volatile() to UserSettings.mock()

Given it now has a userID parameter with a default value..

UnitTests/Sources/AnalyticsSettingsScreenViewModelTests.swift

@@ -23,7 +23,7 @@ final class AnalyticsSettingsScreenViewModelTests {
         client.isRunning = false
         analytics = AnalyticsService(client: client, appSettings: appSettings)
         
-        viewModel = AnalyticsSettingsScreenViewModel(userSettings: UserSettings(appSettings: appSettings, accountSettings: .init()),
+        viewModel = AnalyticsSettingsScreenViewModel(userSettings: appSettings.userSettings(for: "@me:matrix.org"),
                                                      analytics: analytics)
         context = viewModel.context
     }