Boost: Ensure Home URL is always a predefined Cornerstone Page

[LiamSarsfield]

Fixes HOG-195: Do not count the homepage as a cornerstone page for plan limits

Fixes HOG-204
Fixes HOG-215

Proposed changes:

• Ensures the Home URL is not included within the user's Cornerstone Page plan.
• Adds a new "Predefined" list of URLs (currently only homepage) that the user cannot edit.

Other information:

• Have you written new tests for your changes, if applicable?
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Does this pull request change what data or activity we track or use?

Testing instructions:

• Go to '..'

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 49 files. Only the first 5 are listed here.

File	Coverage	Δ%	Δ Uncovered
projects/plugins/boost/app/assets/src/js/features/cornerstone-pages/meta/meta.tsx	0/73 (0.00%)	0.00%	6 💔
projects/plugins/boost/app/lib/cornerstone/class-cornerstone-utils.php	1/22 (4.55%)	4.55%	1 ❤️‍🩹
projects/plugins/boost/app/data-sync/class-minify-excludes-state-entry.php	1/10 (10.00%)	10.00%	-1 💚
projects/plugins/boost/app/lib/class-super-cache-tracking.php	1/2 (50.00%)	50.00%	-1 💚
projects/plugins/boost/app/lib/critical-css/class-critical-css-state.php	1/99 (1.01%)	1.01%	-1 💚

Full summary · PHP report · JS report

_{Coverage check overridden by I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage. .}