Business Hours Block: allow users to indicate that they're open "24 hours" per day (always open)

This is directly related to #16622. We misunderstood the Jetpack user's request. If a business is **always open**, you can't easily indicate that. Instead, you have to add hours like these:

Image Link: https://d.pr/i/jPqvqS

It'd be nice if there was an option for "24 hours per day" or something equivalent, as that'd be easier to understand at-a-glance.

Start by reviewing #16622 and the existing Business Hours Block implementation; the issue does not name a file or test. Define the UI behavior for an always-open business, then verify that users can select an equivalent of “24 hours per day” and that the schedule is represented correctly.

Written by the indexing model from the issue text.