Follow up for #1951268: Convert /forum and /forum/% to new routing system, convert Forum to subclass Term entity type
Problem/Motivation
#1951268: Convert /forum and /forum/% to new routing system, convert Forum to subclass Term entity type will add a primitive form controller for forum module. This issue is to flesh it out and remove forum_form_forum() and forum_form_container() to instead use a first-class entity form controller, moving the menu callback to the new routing system in the process.
Proposed resolution
Flesh out ForumFormController to replace forum_form_forum() and forum_form_container()
Remaining tasks
Write patch
User interface changes
None
API changes
forum_form_forum(), forum_form_submit() and forum_form_container() removed.