Provide a way to get source for a snap in the store

[Lin-Buo-Ren]

F.Y.I.

Proposal: Allow exposing the public source repository that is used to build a particular snap in the store page

Rationale If a certain snap is set up to be built and published by one of the Canonical controlled build services (i.e. build.snapcraft.io and Launchpad) it might be beneficial if the corresponding public source repository (and even the build logs) can be exposed on the store page so that the user can know where it is come from(increases the transparency). Implementation Details If the snap’s automatic publishing is set up we should also disable the ability for the publisher to publish the snap…