Implement new Homebrew Cask install behavior

[niksy]

Information

Warning: The default Caskroom location has moved to /usr/local/Caskroom.

Please migrate your Casks to the new location and delete /opt/homebrew-cask/Caskroom,
or if you would like to keep your Caskroom at /opt/homebrew-cask/Caskroom, add the
following to your HOMEBREW_CASK_OPTS:

  --caskroom=/opt/homebrew-cask/Caskroom

For more details on each of those options, see https://github.com/caskroom/homebrew-cask/issues/21913.

References

• How to migrate to the new location? Homebrew/homebrew-cask#21913
• Use legacy Caskroom if it exists, but warn of change Homebrew/homebrew-cask#21901
• Change artifact behavior to moving instead of symlinking Homebrew/homebrew-cask#13966
• Changes to homebrew-cask installation behaviour Homebrew/homebrew-cask#13201

Best approach

• Uninstall every application (not zap!)
• Remove HOMEBREW_CASK_OPTS from .extra and .exports since apps will now be moved to /Applications by default
• Install every application

[niksy]

Mentioned in niksy/knowledge@a914db9 so this can be closed for existing installations.