upd: enforce a more strict Form call pattern.

form management is now split into:
- initial generation (typically in doGet)
- actual submitting (typically in beforePost) resulting in
 - an error (permament or non-permament)
 - a submission result
   - redirect
   - success message
   - custom
- re-emitting if needed (typically in doPost)

Change-Id: Ic226bb886a513b6dfbd844294d2092b653c5df5b