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.

Types/Remote.hs

@@ -66,6 +66,8 @@ data RemoteA a = Remote {
 	repo :: Git.Repo,
 	-- a Remote can be assocated with a specific local filesystem path
 	localpath :: Maybe FilePath,
+	-- a Remote can be known to be readonly
+	readonly :: Bool,
 	-- the type of the remote
 	remotetype :: RemoteTypeA a
 }