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/EmojiPickerScreenViewModelTests.swift

@@ -55,7 +55,7 @@ struct EmojiPickerScreenViewModelTests {
         emojiPickerStream = stream
         
         viewModel = EmojiPickerScreenViewModel(selectedEmojis: selectedEmojis,
-                                               emojiProvider: EmojiProvider(userSettings: .volatile()),
+                                               emojiProvider: EmojiProvider(userSettings: .mock()),
                                                continuation: continuation)
     }
 }