Add preference for prerelease updates channel

[johndbritton]

Feature request

Prerelease updates channel

As a user of Teleport, I should be able to tick a box to opt-in to prerelease versions of the application.

The motivation for the feature

As we make changes to the app we want to get feedback from users that are open to trying code that hasn't been rigorously tested. I'd expect only contributors and power users would opt-in to prereleases, but it would be helpful to be able to test deploying new versions with a smaller subset of users.

How the feature would be relevant to at least 90% of Teleport users

It will help everyone get more stable versions of the app.

What alternatives to the feature have been considered

We could just manually install pre-release versions.

[johndbritton]

The stable channel Appcast is served at /appcast/stable.xml and to implement this feature we need to make the app look at /appcast/prerelease.xml instead.