Add a PWA manifest
This should (hopefully) fix the request in this comment by /u/traveler:
https://raddle.me/f/Postmill/155643/-/comment/279925
Adds a manifest.json which specifies the "standalone" display type (also app name and icon).
I wasn't able to get Postmill running properly on my pc so I couldn't test it, I'm so so sorry about that.