Allow admins to set the creation and modification date of content

[davisagli]

This can be helpful when migrating content from another site.

[jensens]

With admin you're talking about the permission cmf.ManagePortal, right?

[davisagli]

@jensens Either that, or we could add a new permission specifically for this use case that is granted to the Manager role by default (and maybe Site Administrator)