Add a beta opt-in to the app preferences

### What

It would be nice to have a way to opt in to the beta, e.g. a Release Channel setting in the app preferences like in the WordPress.com electron app:


Someone who opts in to the beta would then get updates when a new beta version is released.

### Why

Having a beta release channel would make it easier for beta testers to stay up to date (getting updates for new versions, rather than seeking them out and manually installing) and make the betas more visible to users who would like to try them and give feedback.

Start by tracing the app preferences entry point and the existing update flow to determine where a Release Channel choice and beta update source belong. Done means users can opt into beta updates from preferences and receive new beta releases automatically; the issue does not name files or tests, so repository research is required.

Written by the indexing model from the issue text.