Frontend-editable models

This PR adds a templatetag (similar to `show_editable_page_title`) that renders frontend-editable every Django model.

Usage:

```
    {% load placeholder_tags %}
    {%  show_editable_model object "field" %}
```

If the toolbar is not active the field content of the `object` instance is displayed.
If toolbar is active, field content is clickable and if clicked a popup is shown with the changeform of that instance.

 
 * [x] Generic changeform support
 * [x] Documentation
 * [x] ``django-hvad`` support for single field edit
 * [x] Check failing tests
 * [x] Add a list of "safely editable" fields
 * [x] Make it possible to edit multiple fields in the same form
 * [x] Support editing of single ``Page`` fields (discarded)