Unpublishing an entire snap should have a confirmation dialog

[Simon Fels]

Unpublishing a snap via the global "unpublish" button in the store UI should never be a one-click action to prevent the user from pressing it accidentially (like I did this morning) and having all revisions of a production-critical snap not available anymore.

Pressing the unpublish button should lead the user to some kind of confirmation dialog where he has to acknowlege he understands the consequences and explains what this really means.

[Shawn Wang (shawn111)]

1. snap foo is released rev 2 for beta channel, if there are rev 1 and 2.
2. press "unpublish" button in the store UI, snap foo would be unpublish.
3. If we want to publish foo again, we can find release button for foo rev 1 or 2.
4. press release for rev 2, there is a dialog to ask which channels for release. If we just selected beta. The snap state is still "Ready to publish".

 >>> The behavior would confuse users.

4*. if we select other new channels, the snap state would be change to "Published"

 >>> This is what we expected.

[Shawn Wang (shawn111)]

In clickapp_forms.py, ReleaseUploadForm.

It limits "Released channels cannot be removed".

It is also the issue of behavior.

The below code change could solve the issue, need to discuss with other store guys. :)

https://pastebin.canonical.com/175312/

[Shawn Wang (shawn111)]

#1, #2 should be commented on https://bugs.launchpad.net/software-center-agent/+bug/1654961 not this one.