Self-hosted site user management

[crazytonyli]

The majority of code in this PR is taken from #23572. See the first comment.

I added some minor changes in the following two commits: Removing a couple of global static variables and adding a feature flag for the new User Management feature.

I will make more changes to fine-tune both the UI and the implementation.

Regression Notes

1. Potential unintended areas of impact

2. What I did to test those areas of impact (or what existing automated tests I relied on)

3. What automated tests I added (or what prevented me from doing so)

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[dangermattic]

	3 Warnings
⚠️	Modules/Package.swift was changed without updating its corresponding Package.resolved. Please resolve the Swift packages as appropriate to your project setup (e.g. in Xcode or by running swift package resolve).
⚠️	This PR is larger than 500 lines of changes. Please consider splitting it into smaller PRs for easier and faster reviews.
⚠️	This PR is assigned to the milestone 25.6. This milestone is due in less than 4 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

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[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr23768-f656ec1
	Version	25.4.2
	Bundle ID	org.wordpress.alpha
	Commit	f656ec1
	App Center Build	WPiOS - One-Offs #10986

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr23768-f656ec1
	Version	25.4.2
	Bundle ID	com.jetpack.alpha
	Commit	f656ec1
	App Center Build	jetpack-installable-builds #10026

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.