Replace nested forms with form objects

accepts_nested_attributes for is a poor solution to the nested objects in
forms problem so replacing that functionality, along with the nested_form gem,
is a must.

* Add app/forms directory and autoload it
* Remove nested_form
* Replace all instances of nested_form_for with form_for
* Tighten up the permissions for strong params being used
* Remove accepts_nested_attributes_for in the topic model
* Add NewTopic and PrivateTopic form objects
* Remove nested attributes from setup controller
* Delete form partial that's only used once.

Misc:

* Standardize form object naming as `form` and not `f`
* Remove `multipart: true` html options for the forms
* Remove unnecessary `.topic_form` class on form tags
* Chill with the nested partials