Add an environment configuration option to make `web_asset_store.xml` use HTTPS

[grantfitzsimmons]

https://github.com/specify/web-asset-server/blob/master/views/web_asset_store.xml

This would switch HTTP to HTTPS

web-asset-server/server.py

Lines 356 to 361 in ae6c660

	@route('/web_asset_store.xml')
	@include_timestamp
	def web_asset_store():
	"""Serve an XML description of the URLs available here."""
	response.content_type = 'text/xml; charset=utf-8'
	return template('web_asset_store.xml', host="%s:%d" % (settings.SERVER_NAME, settings.SERVER_PORT))

Import from the environment variables