(bug 39773) Fix "You cannot post a blank message" error on every summary edit

Standardise the handling of edit form wrappers -- they are all now
added in the show{PostEditing,NewThread,Summarize,Reply}Form
functions instead of in various random places and nested and so
on.

Change-Id: If3e53bb9741e268d678c36777fd2ec1f06008204