Send through a bot token so messages can be deleted again

Incoming webhooks return no message id, so nothing can be removed later.
Settings now take a bot token plus channel, chat.postMessage records channel
and timestamp on the run, and the history carries a delete button per
message. The webhook stays as a fallback and is marked as not deletable.

drizzle/0012_solid_tarantula.sql

@@ -0,0 +1,3 @@
+ALTER TABLE "report_run" ADD COLUMN "channel" text;--> statement-breakpoint
+ALTER TABLE "report_run" ADD COLUMN "message_ts" text;--> statement-breakpoint
+ALTER TABLE "report_run" ADD COLUMN "deleted_at" timestamp with time zone;