Add ImprintPage model

[timobrembeck]

Add an Imprint and ImprintTranslation model which should be similar to Page and PageTranslation.

[Thomas125]

I made a test by creating to test-models one "Oberklasse" and one "Unterklasse" where Unterklasse inherits from Oberklasse.
When I query all Oberklasse-instance I get all instances, no matter which class was chosen on creation.
(When I query all Unterklasse-instances I only get the instances which were explicity created as Unterklasse-Instances, of course)

So, to keep the behaviour that "Page.objects.all()" only returns the "content pages" (not the disclaimer page) we would need something like a "GeneralPage" which is inherited from Page. Analog thing with PageTranslation.

Under these circumstances I find it more sensible to create a DisclaimerTranslation-model with the attributes region, language and text with region and language as composite primary key (or as constraint unique_together).
To handle the feedback, we can add a flag is_desclaimer_feedback to the region-feedback-model, which defaults to False.

[Thomas125]

Does the disclaimer page and its translations have the same translation workflow like the normal pages? Do we also keep older versions?
So do we need the attributes currently_in_translation, version and minor_edit for a disclaimer page?

[timobrembeck]

Oh sorry, it seems that I should have done better research before proposing this.

I found a few ways how to achieve the desired behaviour with a custom model manager, but this is kind of hacky and not worth the drawbacks I think.

So yes, probably it's best to define these as standalone classes... if we use an abstract base class for the content in the future, we can make our optimizations there.

And yes, I'd propose to use the same versioning and translation system as for pages, including all special fields.

[timobrembeck]

One other option would be to define an AbstractBasePage which is the parent model of Page and Imprint and an AbstractBasePageTranslation which is the parent model of PageTranslation and ImprintTranslation.
Maybe, this can also be implemented together with #320 Refactor translation models.