When `solved_topics_auto_close_hours` is null/missing in the DB for
Support type categories, the UI defaults to 48 hours for this setting,
but this can be surprising for admins when changing some other support
category setting, as it will go from 0 -> 48 hours in the background.
This commit fixes the issue by backfilling a new custom field
value for `solved_topics_auto_close_hours`, setting it to `0`
on categories with `enable_accepted_answers` set to true
and no existing value for the field.
New support categories are unaffected, as we set 48 hours by
default there.