This change copies the Wagtail admin favicon.ico (from
wagtail/admin/static_src/wagtailadmin/images) into the Sphinx build
directory so that built docs use the same favicon as the admin.
Makes use of [the Sphinx `html_favicon` setting](http://www.sphinx-doc.org/en/1.5.1/config.html#confval-html_favicon)
which adds this line to the generated HTML:
```html
<link rel="shortcut icon" href="_static/favicon.ico"/>
```
Fixes #4340.
This is where developers expect it to be, similar to Django and other
projects. The version info still exists at the old `wagtail.wagtailcore`
location, for backwards compatibility.
Fixes #2557
Django 1.9 requires this line before importing any models. Autodoc
imported models to get their docstrings, causing errors in the build
process.
Fixes #2014.
