Add twig variables to easily override HTML tags

Problem/Motivation

Some templates do use blocks which let you override the content. However, tags that are defined outside the blocks can't be overwritten.

Proposed resolution

Add twig variables for the most common templates to easily override the html tags. For example in field.html.twig;

...
{% set wrapper_tag = wrapper_tag ?: 'div' %}
...
<{{ wrapper_tag }}{{ attributes.addClass(classes) }}>
  {% block content %}
    ...
  {% endblock %}
</{{ wrapper_tag }}>

Remaining tasks

User interface changes

API changes

Data model changes