[REF] dev/core#5803 CRM_Utils_System::theme - split out maintenance calls

[ufundo]

Overview

A small step to cleaning up strange theme function. Acting on quixotic 10 year old todo message.

Before

• theme function has two additional params for $print and $maintenance
• it's called in 3 places in core with $maintenance = TRUE
• handling for $maintenance = TRUE and $print = [truthy] looks haphazard
• looks like callers to theme expect a return value to echo - but in most cases theme just prints itself and returns nothing

After

• split out calls with $maintenance = TRUE to a separate function with a well-defined signature
• add deprecation notices to unexpected calls to theme

Comments

Inscrutability here didn't help when trying to work on #32398

[civibot]

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[ufundo]

Maybe worth noting: previously calls to theme with $maintenance = TRUE led to exit() being called. That's not forced by renderMaintenanceMessage, which just returns the rendered content. As far as I can see nothing else happens after renderMaintenanceMessage in any of the callers, so they should just exit cleanly anyway.