Fixing enabling and disabling FileDownload

[hoxbro]

Fixes #1510 and #1820

There seems to have been two problems: The first problem is that Button do not have a value property and therefore give the "Uncaught TypeError: t is undefined" as mentioned in #1510 and here.
The second problem was that the change in disabled was not connected to the anchor element.

This is my first time trying to write code in TypeScript, so I think it would be very good if someone with more experience in TypeScript can take a look at it and test that I haven't done something wrong or if there is something which can be done better.

An example of the fix in action can be seen below:

from io import StringIO

import param
import panel as pn


class Download(param.Parameterized):
    enable = param.Action(lambda self: self._enable())
    disable = param.Action(lambda self: self._disable())

    def __init__(self, **params):
        super(Download, self).__init__(**params)
        self.fd = pn.widgets.FileDownload(callback=self.cb, filename='data.txt')
        self.fd.disabled = True

    def cb(self):
        return StringIO('foobar')

    def panel(self):
        return pn.Column(self.fd, *pn.Param(self.param, show_name=False))

    def _enable(self):
        self.fd.disabled = False

    def _disable(self):
        self.fd.disabled = True


dl = Download()
dl.panel().servable()