Themes: Alert user if updates for themes on Jetpack sites are available

[lancewillett]

Idea is to show when themes have updates available (and have a way to update them like plugins on Jetpack sites do now in Calypso).

Via @rachelmcr's feedback

For example, when I’m in my self-hosted dashboard I see a notice:

Same as with plugins, we could trigger theme updates without building the install flow yet.

[stale]

This issue has been marked as stale because it hasn't been updated in a while. It will be closed in a week. If you would like it to remain open, can you please comment below and see what you can do to get things moving with this issue? Thanks! 🙏