This commit introduces a simple field where the user can now specify custom
HTTP headers. This commit does not introduce an improved system for storing
HTTP headers as fixtures - such a change would modify both the existing unit
tests as well as this devtool.
This commit adds a new developer tool: The "integrations dev panel"
which will serve as a replacement for the send_webhook_fixture_message
management command as a way to test integrations with much greater ease.
