This branch enables mailbox support in release builds.
I've renamed the feature flag to deliberately break compatibility with briar-desktop, to ensure we don't accidentally enable support in briar-desktop's UI. We should add a separate flag to DesktopFeatureFlags for the UI.
I suggest we set shouldEnableMailboxInCore() to true in briar-desktop even if the mailbox UI isn't ready yet, so that communication between desktop and Android can benefit from the Android device's mailbox (if any). If we're going to do this soon then we could remove the core feature flag entirely instead of renaming it, as the flag will be true on all platforms.