Add "preview" update channel to match macOS

Currently we only have a "stable" update channel. It would be worthwhile to have a preview channel for dogfooding or customers that want to try new features before they're released.

- We will need to generate version numbers for preview builds
- macOS currently does this by setting the 4th version field (`0.0.0.z`) to the number of commits since the last release
- We will need to package them identically to release builds
- And sign them!
- We will need to push the preview artifacts somewhere
- macOS currently does this with a "preview" release, but I dislike this pattern and would probably opt for storing them on `releases.coder.com`
- We will need to update the `SparkleUpdateController` code to handle channel selection, and add UI elements to select a channel

One potential challenge is that our installers forbid downgrading (for good reason). Once a user switches to the preview channel, they won't be able to switch back to the release channel until a new version is released (or they uninstall/reinstall). I think NetSparkle probably handles this for us though if we go with the same version number system as macOS.