add configuration for macos notarizing

[nikolaiwarner]

This should make it so when we pass builds around users will not be greeted with security warnings during install. Also I updated the macos dmg with a custom background.

[cblgh]

awyisssss 😱 📦

in the medium term, i wonder if we can configure cabal-desktop builds to be built by Travis (or Github Actions) like cabal-cli

[nikolaiwarner]

@cblgh at the moment travis is set up to make builds when we push a version tag and those builds go directly to a draft github release.

however, now that we're notarizing, i need to do some more config on travis to have it do that for us as well.

[cblgh]

ah nice, i thought you were pulling those together by hand ^^