* Send payload in application/json for outgoing webhooks
The Add outgoing webhook UI now has a 'Content-Type' field that allows
to choose between application/x-www-form-urlencoded and
application/json. All outgoing webhooks created before this change will
be considered as x-www-form-urlencoded. There's also a minor change in
the way the outgoing webhook summary is displayed: the 'Callback URLs'
label was missing.
* Fix JS formatting errors
* Increase ContentType field length to 128
* Renamed Commands to Slash Commands in backstage UI
* Updated displayed info for listed commands and outgoing webhooks
* Disallowed empty outgoing webhook trigger words and improved client-side validation for them
