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.

Remote/Git.hs

@@ -98,6 +98,7 @@ gen r u _ = new <$> remoteCost r defcst
 				then Just $ Git.repoPath r
 				else Nothing
 			, repo = r
+			, readonly = False
 			, remotetype = remote
 			}