Refactors the flask server and other audio/video streaming infrastructure into a new module.

[xsdg]

This required jumping through a lot of hoops because Flask is not
object-oriented, and is intended to be implemented at a module level, which
makes it completely un-testable. I opted for a static class implementation,
which still provides some encapsulation and can be tested with the addition of
a reset function.

That said, a better option would be to switch over to something like
https://github.com/pallets-eco/flask-classful , which provides a
properly-encapsulated Flask implementation. Ideally, this will behave more like
Java servlets, which are usually pretty straightforward to test.

Tested

On Debian Linux

# Played audio via pulseaudio with parec backend
$mkchromecast --debug

# Played audio via pulseaudio with ffmpeg backend
$mkchromecast --debug --encoder-backend ffmpeg

# Played video with ffmpeg backend
mkchromecast --video --debug -i <file.mkv>

This is part of #434