use fetch request to upload form data

[rmorshea]

closes: #574

This change prevents the default behavior of a <form> element's onSubmit event handler. Instead of navigating away from the current page to submit form data, it is sent to the server via a Javascript fetch request. We do not push form data into the URL (this behavior will be left to third party packages like idom-router).

Improving form submission behavior will allow users to use form elements to send, among other things, file data to servers. For example, the following will now be possible:

from idom import component, html
from idom.backend.starlette import configure
from starlette.applications import Starlette

app = Starlette()

async def handle_file_upload(request):
    form = await request.form()
    ...  # do stuff with the form

app.add_route("/file-upload", handle_file_upload, methods=["POST"])

@component
def Example():
    return html.form(
        {
            "enctype": "multipart/form-data",
            "action": "/file-upload",
            "method": "POST",
        },
        html.input({"type": "file", "name": "file", "id": "file-input"}),
        html.input({"type": "submit", "id": "form-submit"}),
    )

configure(app, Example)

Checklist

Please update this checklist as you complete each item:

• Tests have been included for all bug fixes or added functionality.
• The changelog.rst has been updated with any significant changes.
• GitHub Issues which may be closed by this Pull Request have been linked.

[rmorshea]

cc: @Archmonger thoughts on this?

[Archmonger]

Is there any way to automate this route handling with some combination of Context scoping and uuid session uniqueness?

Ideally these route would get automatically created within from idom.backend.my_chosen_backend import configure.

See here.

---

Also, how would "signaling back" to IDOM work in this scenario?

[rmorshea]

After thinking further, modifying the default behavior of forms might not have been the best idea. Doing so should probably be the purview of third-party packages (e.g. idom-router a la React Router's forms).