Customizer: Add "customization locking"

[melchoyce]

Let's introduce locking to the Customizer, so it's harder to accidentally overwrite someone else's concurrent changes. This is especially important when multiple people are collaborating on building the same site.

See attached mockup.

Related: #31897, #31436

(Just a note, "user" here is the user's display name)

@melchoyce what about for the other user who gets their session taken from them? What should be shown to them?

Good question, especially since we don't have revisions. Let me think on it.

Okay... since we don't have revisions yet, what if we disallowed folks from taking over for now? Could do something like:

[User] is already customizing this site. Please wait until they are done to try customizing.

Work in progress PR: ​https://github.com/xwp/wordpress-develop/pull/269

I'm blessing this because this is an important enhancement for a major feature in the release (#39896), and users would experience defects otherwise when stomping on each other's changes.

In 41839:

Customize: Add changeset locking in Customizer to prevent users from overriding each other's changes.

• Customization locking is checked when changesets are saved and when heartbeat ticks.
• Lock is lifted immediately upon a user closing the Customizer.
• Heartbeat is introduced into Customizer.
• Changes made to user after it was locked by another user are stored as an autosave revision for restoration.
• Lock notification displays link to preview the other user's changes on the frontend.
• A user loading a locked Customizer changeset will be presented with an option to take over.
• Autosave revisions attached to a published changeset are converted into auto-drafts so that they will be presented to users for restoration.
• Focus constraining is improved in overlay notifications.
• Escape key is stopped from propagating in overlay notifications, and it dismisses dismissible overlay notifications.
• Introduces changesetLocked state which is used to disable the Save button and suppress the AYS dialog when leaving the Customizer.
• Fixes bug where users could be presented with each other's autosave revisions.

Props sayedwp, westonruter, melchoyce.
See #31436, #31897, #39896.
Fixes #42024.

In 41919:

Customize: Fix visibility of overlay notifications in Safari.

Props sayedwp, westonruter.
Amends [41390].
See #35210, #42024, #42052.

@westonruter please update the error message to match other ​permission error messages. See my patch.

Related: #34521

In 41953:

Customization Locking: Update error message to use existing strings.

Props ramiy.
See #42024.

Thanks!