Fix checkbox JS regression for import data selection and email new te…

[larssandergreen]

Overview

Regression from #26598. That PR changed the signature of add() for a checkbox, breaking these two checkboxes which no longer had their onchange added as it was in $extra, which is now ignored.
These are the only two that show up by searching.

Before

1. On Contact - New Email, clicking the Save as New Template doesn't show the template name field.
2. In Import, if you add a data file, go to the next step, then go back, the Use data already uploaded checkbox doesn't show/hide fields below it as intended.

After

Show/hide works as intended.

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[larssandergreen]

Jenkins test this please

[eileenmcnaughton]

test fail looks unrelated - test this please

[colemanw]

retest this please

[colemanw]

Gives me a warm feeling to know that, although the rest of the world has shunned it, we at CiviCRM still have room in our hearts for that old onclick html attribute.

[demeritcowboy]

I don't know if there'll be another 5.64 release but might be worth a backport?