Slack Incoming webhooks is a legacy custom integration - an outdated
way for teams to integrate with Slack. These integrations lack newer
features and they will be deprecated and possibly removed in the
future. Slack team do not recommend their use. Instead, it's suggested
to use Slack apps.
Add and use _readaccountconf_mutable_default and
_saveaccountconf_mutable_default helpers to capture
common default value handling.
New approach also eliminates need for separate
underscore-prefixed version of each conf var.