[snaps] Make upload process more easier to follow

[Daniel Holbach]

The upload process for a snap for me was as follows:

1) I ran into bug 1572399 - it'd be good if either snapcraft or the store would explain where and why to register the name.

2) I had to set the release channel of my snap, which wasn't very obvious. A new developer won't know what this is about and what to set it to.

3) I had to publish my app. This was very confusing. At the top of the app page it said "Package status is published". In the left hand revision pane there was a 🗸 next to the last upload, so I thought everything was fine. The last thing I looked at was the button at the very bottom which said "published".

The steps should be a lot more obvious.

The docs should also be updated once bug 1326875 is fixed.