Extjs index template

I've finally took time to rework ExtJS HTML generating to templates - where it belongs. I can now simply customize generated HTML without any limitations.

The second patch allows me to define "extjs/index.html" template in my project dir and inherits "extjs/index.html" in the lino.modlib.extjs module by specifying 
```
{% extends "lino.modlib.extjs:extjs/index.html" %}
```