@root.submitted may not exist in flow_reply_form

For no-js version of flow-reply, templateHelper::post()
doesn't feed submitted key to the template, which causes
a warning.

Change-Id: Ibb2e643f795af2745a6abcd759608fc11a9d71b1
(cherry picked from commit 80199234a496bab4c0cf25ddb5a3adfa7b8dfa3d)