[iOS] Run save/delete tasks as background actions

[amdilley]

When saving or deleting posts in the application, you are prompted to "Keep your phone unlocked and don't switch apps". For initial save/delete steps, this can take a long time, especially for moderate to heavy users. Given that the first actions will leave the biggest impressions on newly onboarded users, it's crucial this is as painless as possible. Running long actions in the background frees them to use their phone and return only when needed.

[micahflee]

Documentation for iOS background tasks: https://developer.apple.com/documentation/backgroundtasks