Update create logic to ignore signed marks unless in sunrise

This addresses an issue where the existing logic assumed that the presence of a
signed mark means the current flow is a sunrise/sunrush request, when this isn't
necessarily true. It's safe to ignore signed marks in other circumstances.

This is a combination of work by Justin Graham <justin.af.graham@gmail.com>,
Nick Felt, and me (Ben). It is based on the original PR located at:
https://github.com/google/nomulus/pull/41

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=140784461

docs/flows.md

@@ -610,7 +610,6 @@ An EPP flow that creates a new domain resource.
 
 *   2002
     *   Service extension(s) must be declared at login.
-    *   Signed marks are not accepted in the current registry phase.
     *   The current registry phase does not allow for general registrations.
 *   2003
     *   Fees must be explicitly acknowledged when performing any operations on a