Add the ability to set how many episodes should be downloaded

[jkmassel]

Expected behavior

The app has a global setting (for bonus points: override-able on a per-podcast basis) for how many recent unlistened-to episodes of a podcast to download.

See: https://forums.pocketcasts.com/forums/topic/possible-to-keep-the-oldest-2-unlistened-episodes-of-a-podcast-autodownloaded/?view=all#post-1069

Actual behavior

The app doesn't currently have this feature.

Steps to reproduce the behavior

Tested on [device], iOS [version], Pocket Casts [version]