Pages List: Allow Changing Homepage Directly

[Aurorum]

Fixes #64072

Proposed Changes

Ensures users are now able to change their default homepage directly from the Pages screen - as mentioned in the original issue - which makes it significantly less confusing for them.

Before:

The card linked to the site URL, but there was no real way to guess this.

After:

Folded	Expanded

Testing Instructions

• Go to Settings > Readings and change "Your homepage displays" to "Your latest posts" (this message won't display otherwise)
• Go to /pages for that site in Calypso
• Verify that the card appears, and that you're able to expand it
• Confirm that you can successfully update your homepage from within this screen

Pre-merge Checklist

I don't have access to these P2s, so I can't see if I'm missing anything, but I have tested this on a Simple and Atomic site.

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?

cc @simison @Copons

[Aurorum]

@tyxla This is only very barely Settings-related, but wonder if I might be so sneaky and ask you to take a look at this whenever you have a spare moment - I've tried to simplify the code a bit, and would love to hear your feedback. :)

[tyxla]

@tyxla This is only very barely Settings-related, but wonder if I might be so sneaky and ask you to take a look at this whenever you have a spare moment - I've tried to simplify the code a bit, and would love to hear your feedback. :)

Thanks for the ping and the good work @Aurorum! Left some feedback in the #64072.