Cron tab: Improve UI for rendering scheduled crons

[nightnei]

Fixes DOTCOM-15893

Proposed Changes

I added cronstrue lib to render human-readable cron times.
Also added description to be explicit that the specific time of execution is randomized.

Why are these changes being made?

While creating new cron - we send "Daily' etc, but backend stores it as raw cron format. After discussion with implementors of the API - it seems that we had to try to do our best to make UI good with the state of things which we have right now.
The discussion - p9o2xV-5WN-p2#comment-12339
I was thinkign about what we can do and I propose to go with the first step as:

1. Explicitly add description to the creation form (UI can be improved later) that the time is randomized
2. Render human-readable information about raw cron times
   See screenshots below.

As the second step, I am going to continue the conversation (p9o2xV-5WN-p2#comment-12339) and try to convince teh team that it will be more rational if they store the requested format at teh server, alongside the command and raw cron time. Hopefully that they will agree and we will make UI much more better. But ATM, what I proposed, I suppose is teh best option, what we can do.

Testing Instructions

1. Create Atomic Site
2. Open site "Setting" tab -> "Cron" -> "Add scheduled cron"
3. Assert that you see additional description
   
4. Add new cron job
5. Assert that you see raw cron tiem and also human-readable description
   

[github-actions]

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[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• agents-manager
• help-center
• notifications
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug improveUiForRenderingScheduledCrons on your sandbox.