Get 6.0 working

This addresses most of #1:

• updated debian/copyright for xpra
• excluded more stuff from upstream that we do not use (packaging and some duplicate files)
• investigated some long lines and justified them in debian/source/lintian-overrides
• fixed a few genuine things caught by lintian (duplicated README.md and misplaced dbus policy)

The big thing, though, is that I removed the html5 client. I don't like doing that, but:

• The "source" that upstream provides for this is not the actual source
• Upstream vendors some very old, possibly unmaintained JS libraries
• It includes a compiled version of simple-keyboard, which has a package-lock.json weighing in at 28631 lines.

I don't think it makes sense to block upgrading to 5.06.0 because of this. Upstream decoupled this from xpra proper, and I think it'd be best to follow suite. The dependencies on these packages will probably change faster than we could keep up with, making a Debian policy-compliant packaging impossible.