Upload macaroons should allow channel restrictions (eg. by regex)

[Michael Hudson-Doyle]

Now that channels have tracks and branches, it would be good to be able to create a macaroon that allows publication to channels matched by regex, e.g. I was thinking about setting up a CI-type system that could push per-PR builds of a snap to edge/test-build-for-pr-NNNN but I would prefer such a system to not be able to push to stable :)

[Adam Collard (adam-collard)]

Let's allow simple globbing, and not a full regex

[Tom Wardill (twom)]

To QA this:

1. Request a `package_release` macaroon, restricted to a channel using an `fnmatch` glob.
2. Attempt to release a snap using this macaroon to a channel that does not match the restriction. It should be rejected with a useful error message.
3. Attempt to release the snap to a matching channel, this should be allowable.