Ability to inject custom html elements in the head element

[fabito]

Problem

There is no (SSR) way to inject custom elements in the head when extending the index.html jinja2 template

Solution

Add a new jinja2 block extend_head to the index.html template to improve customizability. For example:

{% extends "index.html" %}

{% block extended_head %}
<link rel="stylesheet" type="text/css" href="./extend/static/extend.css?v={{ version }}" media="screen">
{% endblock extended_head %}

Test

This is how the extend_web_ui example looks like with the new table styling:

[cyberw]

LGTM. One small thing, could you maybe add a comment describing what the extra css actually does? Or perhaps just name the file something more to the point instead of extend.css?

[fabito]

Sorry I couldn't think of a more interesting example...