feat: Make frontend plugins re-usable components

[fsbraun]

When designing frontends it is key to avoid repeating template/HTML code. This PR makes djangocms-frontend components like alerts, cards, buttons or links, etc.

This PR provides a simple way to re-use djangocms-frontend plugins in templates:

{% load frontend %}
{% url "some-name" as link_target %}
{% plugin "link" external_link=link_target link_type="btn" link_context="primary" link_outline=False %}
    Click me!
{% endplugin %}

or

{% load frontend %}
{% plugin "card" card_alignment="center" card_outline="info"  card_text_color="primary" card_full_height=True %}
    {% plugin "cardinner" inner_type="card-header" text_alignment="start" %}
        <h4>Card title</h4>
    {% endplugin %}
    {% plugin "cardinner" inner_type="card-body" text_alignment="center" %}
        Some quick example text to build on the card title and make up the
        bulk of the card's content.
    {% endplugin %}
    {% plugin "listgroupitem" %}An item{% endplugin %}
    {% plugin "listgroupitem" %}A second item{% endplugin %}
    {% plugin "listgroupitem" %}A third item{% endplugin %}
{% endplugin %}

These plugins are not created in the database. They work with djangocms-frontend plugins and most other django CMS plugins.