'snap list' should show installed and subscribed channel

[Mark Shuttleworth]

It is possible to subscribe to a channel but not get anything from that, reverting to something more stable. The listing should show both the actually-installed channel in parenthesis (brackets) after the snap name[@developer] and the subscribed channel in a Channel column.

  $ snap install foo --beta
  foo (beta) 1.1 installed
  $
  $ snap install bar@jamie --candidate
  bar@jamie (stable) installed while candidate channel is closed
  $
  $
  $ snap list
  Name Version Rev Channel
  foo (beta) 1.1 12
  bar@jamie 2.3 37 candidate
  $

Note, both the subscribed channel and the actually-installed version in brackets after the name@developer are shown when not stable.

[Michael Vogt (mvo)]

Thanks for the bugreport.

To make this fully work some more steps are required, however I started with a strawman branch that implements the UI and I pushed it to https://github.com/snapcore/snapd/compare/master...mvo5:feature/snap-list-channels?expand=1 so that I can validate that I understood the UI bits correctly.

With that, the following is displayed:
"""
Name Version Rev Developer Channel Notes
android-tools 5.1.1r36+git20160322-1 1 canonical edge devmode
docker 1.11.2-9 56 canonical stable devmode
hello-mvo (beta) 2.10 1 mvo edge -
"""

I.e.:
- android-tools tracks the edge channel and that channel is open. in this case no "(edge)" is added as this channel is what the user asked for
- docker tracks the stable channel and that channel is open, no (stable) here too
- hello-mvo tracs the edge channel, however there is no snap in the edge channel so it falls back to beta and this is displayed via the "(beta)" behind the name.

If that looks good I will move forward and propose this branch. If not, I will of course fix as needed.

What we will also need to do:
- display when a snap is installed from a more stable channel in `snap install`
- check with the store to ensure we get the right information, i.e. when asking for "hello-mvo/edge" we need to get information back that we actually got a snap from the "beta" channel.

[Michael Vogt (mvo)]

One concern that came up during the team discussion about this is that we probably want to tweak the UI a bit further to making parsing via e.g. scripts easier.