When saving the Allow topic owner and staff to mark a reply as the
solution
field, which is a category custom field, because "true" was stored as
a string in the DB then coverted to a boolean for FormKit checkboxes,
when we saved it again the value was later converted to "t" in the
database instead of "true".
We can make the FormKit checkbox a bit more lenient of truthy
values, and also make sure these booleans are converted to string
on the server-side for category custom fields too.
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Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>