Internal server error when clicking on "Settings"-button.

[jrief]

How to reproduce:

• Create a customized page content admin inheriting from PageContentAdmin.
• In method get_fieldsets, add some extra fields part of your page content extension model.
• Open a page with more than one version.
• From the toolbar, open the dialog to "Manage Versions…".
• Locate an unpublished version and click on the "Settings"-Button.

This will raise a ValueError in cms_config.py:283 stating that list.remove(x): x not in list.

The problem could be easily fixed by checking if f_name is member of fields.

Shall I create a pull request for this? I however don't know how to create a test for it.

[fsbraun]

@jrief Thank you for offering a PR!