Assign default add patch set permission on refs/for/*

By default each user should be able to add patch sets on all open changes.
We have created a new permission to lock changes, such that only
the change owner or a specific group can add new patchsets to exisitng
changes.

project.config

diff --git a/project.config b/project.config
index 9e427f3..1659d51 100644
--- a/project.config
+++ b/project.config
@@ -60,3 +60,5 @@
 [notify "team"]
 	email = upspin@google.com
 	filter = -message:do-not-review
+[access "refs/for/*"]
+	addPatchSet = group Registered Users