Meta-description for pages/events/pois

[timobrembeck]

Motivation

To improve SEO, we want to add the possibility to add meta-descriptions for pages/events/pois.

Proposed Solution

• [ ] Add a new field meta_description = models.TextField(blank=True) to AbstractBasePageTranslation, EventTranslation and POITranslation models.
• [ ] Add this field to the PageTranslationForm, EventTranslationForm and POITranslationForm.
• [ ] Add a new textarea beneath the text/description fields to the templates of that form (page_form.html, event_form.html, poi_form.html).
• [ ] Add description property to the Region model, which can be used for the content overview in the app.