Refine slack integration to be a module/plugin

[bitsofinfo]

Refine slack integration to be a module/plugin or just cleanup the configuration specification to get rid of references to "slack" as it can really be used to send data to any endpoint.

[bitsofinfo]

The following methods should delegate to a HTTP abstraction:

event_handle_trigger_pipeline
event_handle_manual_choice
event_handle_notify
event_handle_respond