This splits up the site title for the Site Health tabs to make it more obvious what page a user is viewing, and which tab under that page. It also removes a possible source of warnings if a user tries accessing a tab that does not exist.
The introduction of this check for existing tabs also improves the accuracy of the active-tab indicator, ensuring the indicator shows up for the default tab, when no other tab has been selected.
Follow-up to [50764].
Props audrasjb, swissspidy.
Fixes#53535.
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