Webpack5 addons status

[thadk]

Stacked on #89 and rebased on #90

Just a proof of concept for addon forms at this point, will refine later in the day after we talk about it.

[thadk]

Today's progress:

• Top addon has a JSON-Schema parameter set in Django but documentcloud django doesn't know about how to read that yet so fails to find "name" in the addon parameters itself and isn't prepared for name to come in as a result of valid json-schema data.
• Bottom addon has the original parameter set in Django. If you click the "simple" form, and submit it, everything seems to work. Worth trying on Staging, though I didn't have time to backport the things to webpack 4 yet.
• Make dispatch Button gray out when there is invalid input as it did before. Right now it does respond when you click it without filling a last name on the first addon.

Top addon JSON-schema parameter:

{"type": "object", "title": "A registration form", "required": ["name", "lastName"], "properties": {"age": {"type": "integer", "title": "Age"}, "bio": {"type": "string", "title": "Bio"}, "name": {"type": "string", "title": "First name", "$svelte": {"field": {"props": {"autofocus": true}}}, "default": "Chuck"}, "random": {"type": "number", "title": "Random number"}, "lastName": {"type": "string", "title": "Last name"}, "password": {"type": "string", "title": "Password", "$svelte": {"field": {"props": {"type": "password"}}}, "minLength": 3}, "telephone": {"type": "string", "title": "Telephone", "minLength": 10}}, "description": "A simple form example."}

Same parameter for top addon prettified:

{
    "type": "object",
    "title": "A registration form",
    "required": [
        "name",
        "lastName"
    ],
    "properties": {
        "age": {
            "type": "integer",
            "title": "Age"
        },
        "bio": {
            "type": "string",
            "title": "Bio"
        },
        "name": {
            "type": "string",
            "title": "First name",
            "$svelte": {
                "field": {
                    "props": {
                        "autofocus": true
                    }
                }
            },
            "default": "Chuck"
        },
        "random": {
            "type": "number",
            "title": "Random number"
        },
        "lastName": {
            "type": "string",
            "title": "Last name"
        },
        "password": {
            "type": "string",
            "title": "Password",
            "$svelte": {
                "field": {
                    "props": {
                        "type": "password"
                    }
                }
            },
            "minLength": 3
        },
        "telephone": {
            "type": "string",
            "title": "Telephone",
            "minLength": 10
        }
    },
    "description": "A simple form example."
}