Draft: Give users more control over extension updates

This MR:

• adds an option to disable automatic extension updates
• exposes it in the Extensions app
• adds an action in the app to explicitly check for updates

I'm marking it as a draft because it needs design work:

• there's a risk of extensions not getting updated at all
• the "automatic updates" and "check for updates" actions both look like options
• possible feature creep (for example updating individual extensions)

That is, the MR is mostly about "parking" the code somewhere upstream, so it doesn't get lost 🤷