Convert render_charts to simple tag for jinja2 support

[RealOrangeOne]

This allows the tag to be called easily from jinja2:

from wagtailcharts.templatetags.wagtailcharts_tags import render_charts
from jinja2 import pass_context

# later...
self.environment.globals.update(
    {
        "render_charts": pass_context(render_charts),
    }
)

The template doesn't use any context, but it's passed in anyway for backwards-compatibility (and because it's no real extra work).

Usage of the tag isn't changed, so this is non-breaking.

[saevarom]

Thanks @RealOrangeOne

[saevarom]

This has been released in version 0.6 on pypi