Snap classic deployment

[casperdcl]

Finally have approval (https://forum.snapcraft.io/t/classic-confinement-request-for-dvc/14124/8) so can deploy snaps to the snapcraft store!

• update CI to auto-deploy to snap store (https://snapcraft.io/dvc)
  • --edge <- master
  • --stable <- tags
• manually release 0.80.0 from https://github.com/iterative/dvc/releases/download/0.80.0/dvc_0.80.0_amd64.snap to check that it works

Note from approval:

This snap is using classic confinement. The publisher of this snap has been vetted (https://forum.snapcraft.io/t/classic-confinement-request-for-dvc/14124/7) and this snap is now granted use of confinement: classic. Note, future uploads may require the snap come from a build on our infrastructure (https://launchpad.net/bugs/1657825) so if you haven't done so already, please configure your builds at https://build.snapcraft.io

[casperdcl]

@iterative/engineering feel free to test snap install --classic dvc if you want - should work now

[efiop]

🔥

[casperdcl]

not sure about the post-merge failure (https://travis-ci.com/iterative/dvc/builds/143225559). Just restarted. Also it appears to be part of stage: build rather than deploy - will fix in different PR.