Polish Social Links block

[jasmussen]

This PR aims to address recent feedback, and fixes #17362. In doing so it also addresses some initial feedback from #17267, namey to provide a generic "block selected" style.

Specifically this PR:

• Removes the gray placeholder color, which was intended to provide a "disabled state" for social links that had not been configured.
• It addresses hover issues reported
• While I'm here, it refines the code a little bit, adds comments here and there
• It hides the sibling inserter. Same argument as removing the mover controls, it will be explored separately in context of general child block improvements.
• It makes the focus style for the selected logo button thicker and darker.
• The opacity changes that indicate unconfigured blocks are simplified to always provide full opacity on hover or select.
• It restores and tweaks the "child block selected" outlines, but in a version that is still simpler than the full thing. This will likely benefit from further polish as we seek to make these child block improvements generalized in Nested Blocks: Provide container block prop to absorb child block UI #17267.
• It hides the "dashed outlines for child blocks" as that clashes with their neutralized margins.

Before:

After:

[mapk]

Thanks for working on this, @jasmussen!

• The hover and focus effect on the parent block feel much better overall. 👍
• The focused state of the child block is definitely much more clearer. It still feels a tad bit awkward but works. The thick left border helps tremendously here.

One other note, which can be addressed in another PR is the focused state of the block inserter + icon. Should the outline shape be a circle as in the other block inserters?

In Social block

In other areas of the UI

[mapk]

[jasmussen]

Sweet, swift review. I'll merge now to address the big points you raised, and I'll follow up on the focus rectangle for the block library button in a subsequent pr.