Big rearrangement to add debconf support.

/etc/default/dump1090-mutability is now generated in postinst,
if it doesn't exist, from a template in /usr/share/dump1090-mutability.
Subsequently, the config file is updated in place if debconf-driven
changes are made.

Static HTML/javascript have moved to a "html" subdir to avoid exposing
the template via the webserver.
Changelog update.