Latest Posts: Add the ability to filter by a taxonomy term

[ryelle]

We need to be able to filter the Latest Posts block by a given tag for the developer blog. They want to show only the "dev-notes" tag from make.w.org/core. See WordPress/wporg-developer-blog#30.

I can imagine fetching based on category could also be useful? But we can stick to just tags if it's easier.

The UI for this can be very simple, maybe even just a dropdown for "category" or "tag", and a free text field for the term slug. I don't think this needs to validate anything, it should be clear if it works based on the live preview.