Add script to export current attribute descriptions as JSONSchema

This is a script that converts attribute descriptions to JSONSchema as used in Leipzig. It includes two different modes:

• Default mode is compatible with the format used in huscy
• Raw mode is compatible with the format used in castellum

There are two notable things missing from this representation:

• Translations (for label, help text, and choices labels)
• Filter-specific information such as operators or filter_form_class

I did not add support for Categories yet because they are rarely used.

$ ./manage.py attribute_schema
{
    "properties": {
        "Date of birth": {
            "format": "date",
            "type": "string"
        },
        "Handedness": {
            "enum": [
                "Right",
                "Left"
            ],
            "type": "string"
        },
        "Highest degree": {
            "enum": [
                "No degree",
                "Elementary school",
                "Hauptschule",
                "Mittlere Reife",
                "Abitur",
                "Bachelor",
                "Master"
            ],
            "type": "string"
        },
        "Language": {
            "type": "string"
        }
    },
    "type": "object"
}

$ ./manage.py attribute_schema --raw
{
    "properties": {
        "d5": {
            "description": "Handedness",
            "enum": [
                10,
                11
            ],
            "type": "integer"
        },
        "d6": {
            "description": "Language",
            "type": "string"
        },
        "d7": {
            "description": "Date of birth",
            "format": "date",
            "type": "string"
        },
        "d8": {
            "description": "Highest degree",
            "enum": [
                12,
                13,
                14,
                15,
                16,
                17,
                18
            ],
            "type": "integer"
        }
    },
    "type": "object"
}