Release channels

[philrz]

When considering the scope of #1211, @jameskerr mentioned that at some point we might want to add support for updates via "Channels" (https://www.electron.build/tutorials/release-using-channels.html). This would allow users to express a willingness to opt in to Zui beta releases should we choose to start producing them.

In the meantime, the separate Zui Insiders releases effectively serve as something like a daily rolling alpha/beta test build.