add support for readonly remotes

Currently only the web special remote is readonly, but it'd be possible to
also have readonly drives, or other remotes. These are handled in the
assistant by only downloading from them, and never trying to upload to
them.

Assistant/Threads/TransferScanner.hs

@@ -16,6 +16,7 @@ import Assistant.Alert
 import Logs.Transfer
 import Logs.Location
 import qualified Remote
+import qualified Types.Remote as Remote
 import Utility.ThreadScheduler
 import qualified Git.LsFiles as LsFiles
 import Command
@@ -122,6 +123,7 @@ expensiveScan st dstatus transferqueue rs = do
 				, use $ check Download True
 				)
 		check direction want key locs r
+			| direction == Upload && Remote.readonly r = Nothing
 			| (Remote.uuid r `elem` locs) == want = Just $
 				(r, Transfer direction (Remote.uuid r) key)
 			| otherwise = Nothing