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