Editor: implement aria-modal and aria-hidden for link insertion in classic editor

[audrasjb]

https://core.trac.wordpress.org/ticket/60799

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[sabernhardt]

Nitpick: Should the open function set a variable to use it only once? The close function has $( '#wpwrap' ) with the aria-hidden attribute, though that function does not assign any variables for reuse. The variable might help readability a little.

[joedolson]

I'm inclined to avoid code churn for legacy code blocks like this; I chose to do it this way for consistency with existing patterns in both functions. Could go either way, but it's not clear to me that it makes any practical difference.

[joedolson]

Adjusting in commit to avoid creating a single-use variable, per this comment.

[joedolson]

In r58450