snap: add another channel for alpha versions

[casperdcl]

Snap supports 4 channels, atm DVC is only using

• stable==latest tag (even if alpha), and
• edge==latest master commit.

Should really use

• stable==latest release tag,
• candidate or beta==latest alpha tag, and
• edge==latest master commit (as is).

I'd prefer candidate (beta could be confusing since the current tags have a for alpha rather than b/beta). However, based on https://snapcraft.io/docs/channels#risk-levels, maybe beta is best:

candidate: for users who need to test updates prior to stable deployment, or those verifying whether a specific issue has been resolved.

Releases in candidate are considered almost ready for going into stable, but need some additional real world experimentation before they move forward. Software reaching this stage will typically have passed all available QA and review processes, since users following it expect a high stability level. Should almost never break.

beta: for users wanting to test the latest features, typically outside of a production environment.

Beta is the first level towards the stabilisation of what was before a fast moving stream of changes. Specific projects may have slightly different terminology for such releases (alpha, beta, etc) but all of these are welcome on this risk level. These releases will almost certainly have passed some sort of review and QA, but may still have unfinished parts. Breaking changes are still relatively common here.