Fix EZP-21124: Checkbox custom attributes on custom tags are not stored

JIRA: https://jira.ez.no/browse/EZP-21124

# Description

The checkbox attributes of the custom tags are not stored in the ezxml, as a result, the UI in Online Editor does not take a previous user input into account.

This is fixed by using the correct jQuery method to retrieve the state of the checkbox (needed because of some BC breaks in jQuery...)

# Tests

manual test