A proposal for a receive-only folder type

[calmh (Jakob Borg)]

Syncthing v0.14.50-rc.1

The Syncthing v0.14.50 release candidate one was just released. It includes initial support for receive only folders: https://docs.syncthing.net/users/foldertypes.html#receive-only-folder. Go hammer on it, and lets iron out some bugs before the actual release?