Get wagtail object to render django template variables

[xmedr]

When working on #1177, we made a wagtail-editable model EventNotice that contains a set of messages that would appear on every event detail page. We also wanted to have the message include details specific to the event being rendered. To that end, we started registering a new rich text editor feature within the hooks:

@hooks.register("register_rich_text_features")
def register_ecomment_feature(features):
    """Register the ecomment link type"""
    feature_name = "ecomment_link"
    type_ = "ecomment_link"

    control = {
        "type": type_,
        "label": "E",
        "description": "Ecomment Link",
        "element": "a",
    }

    features.register_editor_plugin(
        "draftail", feature_name, draftail_features.BlockFeature(control)
    )

    from_format_str = (
        "a[href={{event.ecomment_url}}][target=_blank]"
        + "[aria-label='Go to public comment - link opens in a new tab']"
    )
    features.register_converter_rule(
        "contentstate",
        feature_name,
        {
            "from_database_format": {from_format_str: BlockElementHandler(type_)},
            "to_database_format": {
                "block_map": {
                    type_: {
                        "element": "a",
                        "props": {
                            "href": "{{event.ecomment_url}}",
                            "target": "_blank",
                            "aria-label": "Go to public comment - link opens in a new tab",
                        },
                    }
                }
            },
        },
    )

Unfortunately, simply saving the django template variable to the model does not allow it to render correctly when taken back out. Let's do some further research on this. Maybe there's a way we can have from_database_format render a template instead of a straight up string.