Move changing a wiki page's parent page to an own action

This was done under "rename" before, where one does not expect it.

This adds a new permission for changing wiki parent pages.
Remove outdated wiki controller tests, improve cuke

This also moves a wiki step out of general_steps.

Reasons for removing the tests:
* The select field is implemented by Rails - no need for us to test this
* Unassignment is now tested by the cuke