PostAgent Additions

* PostAgent can now send `patch`, `put`, and `delete` requests in addition to the already-possible `get` and `post`.
* PostAgent can use `content_type` of `json` instead of `form`.
* PostAgent can optionally not send in the incoming event, but still use normal Liquid interpolation on `payload`.
* (Unrelated) fix Agent creation view to correctly select an Agent's default schedule.