Start to abstractify Page into a BasePage so that it can be safely subclassed.

Note: Naive reusing the PageAdmin/PageAdminForm with your Page subclasses will not work due to extensions fiddling with the admin classes. Needs more work on that front.

Refs #241.
Move out PageManager into a BasePageManager.
Make sure that ActiveAwareContentManagerMixin does keep its data per-class (and not shared amongst subclasses).
Move registration of default Page request processors into own method so we can reuse that for other BasePage subclasses.

Refs #241.
Update german translations.