Jinja guidelines

A first (and still somewhat incomplete) version of code guidelines for jinja2 templates.
Please, chip in.
Stimme Nico zu, das eskaliert dann schnell zu sowas:
```HTML
                                        <div data-that="{% if view.that -%}
                                                        {{ view.that }}-postfix
                                                        {{%- endif %}"
                                             data-that-really-long-name="
                                             {%- if view.that-really-long-name
                                              %}{{ that-really-long-name }}
                                              -that-really-long-postfix
                                              {{%- endif %}" style="background:
                                                                    url({{ vie\
                                                                           w.g\
                                                                           ive\
                                                                           _me\
                                                                           _ba\
                                                                           ckg\
                                                                           rou\
                                                                           nd_\
                                                                           ima\
                                                                           ge()
                                                                         }}">

```
(Keine Indentingkorrektheits-Garantie :grin: )