Previously, the HTML export template `html.hbs` included CDN links
for the HTML and CSS resources.

This commit enables Webpack to create a new `htmlexport.html` at
build-time, which includes all resources inline.
That template is then used as before by the frontend to be populated
with the rendered note content.

The tradeoff is that each exported .html file is about 5.6 MB in size,
as we need to inline all fonts (icons & emojis).

Signed-off-by: David Mehren <git@herrmehren.de>