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.
diff --git a/project.config b/project.config
index 08a197a..389c996 100644
--- a/project.config
+++ b/project.config
@@ -93,3 +93,5 @@
 [access "refs/heads/master-with-bazel"]
 	push = group Bazel robots
 	push = group mdb/boringssl-commiters
+[access "refs/for/*"]
+	addPatchSet = group Registered Users