Snap: Build snap in CI and publish to the edge channel for builds against master

Merged Ken VanDine requested to merge kenvandine/Remmina:snap_build_ci into master

Publish snaps to the edge channel, but only if the branch is master.

This requires gitlab to have the private variable SNAPCRAFT_LOGIN to be set with the exported key from the following command:

"snapcraft export-login --snaps=remmina - "

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