[v4.0] Refactor to use SimpleForm

Refactor to use SimpleForm

Converted all the internal forms to use SimpleForm rather than our own custom form builder. This provides better consistency with bootstrap forms, as well as well tested API for defining new form inputs. This will primarily affect developers when they create content blocks. New content will be generated using simple_form syntax like so:

<%= f.input :photo, as: :file_picker %>

rather than the older syntax that looks like this:

<%= f.cms_file_field :photo %>

The old form_builder methods like cms_text_field and cms_file_field have been deprecated and will generate warnings when used. These methods are scheduled for removal in BrowserCMS 4.1. It's recommended that custom content blocks be upgraded to use the new syntax when feasible. The deprecation warnings should provide some guideance, but also look at simple_forms documentation http://simple-form.plataformatec.com.br for help.