Fix hiding disabled snapcast section in responsive mode

[jotak]

In responsive mode, the snapcast section takes a lot of space for nothing when it is disabled. I noticed there was already an intent to hide it, as _output-control.scss had a display: none for that, unfortunately it was not targeting the correct class. And also, there was a remaining issue of background still taking the space due to absolute positioning.

So this PR:

• Use flex column rather than absolute position in responsive controls
  The absolute positioning was making it hard to hide the snapcast section
  in responsive mode.

• For flex columns, needed to add enclosing sections in DOM structure

[jotak]

(tips: hide whitespaces while reviewing ... the diff is much easier to read)

[jotak]

Hey @jaedb ,
I hope this PR is ok with you.
I added a second commit with the built files, I don't know if you expect them to be part of the commits, or if it's anyway overwritten during releases or something...

Also: I'm not really a css guy, please let me know if something is done wrong.
Since I'm new to mopidy I haven't fully understood yet what's snapcast and how it works, so I haven't tested my PR when it's enabled. That's surely something to do.

Also, is there a chat channel for Iris? On mopidy's zulip maybe? I'd be interested in contributing more if that's ok, maybe, however there's a couple of things I have some trouble with, like the dev mode...