use named wrapper config for simple_form

This pull request covers:
- BrowserCms content-block forms use a named simple forms config.
- The host app can then use a different default config.

When using SimpleForm in the host app, if the included wrapper config in lib/cms/configure_simple_form_bootstrap.rb is not sufficient, a simple_forms initialiser should be added, and a copy of the browsercms wrapper config element added to that.