Adds 'post to channel' functionality. (#901)

* Adds 'post to channel' functionality. Closes #613

* Add specs
Update Slack integration marketing page

* Fix specs

* 💚

server/presenters/slackAttachment.js

@@ -1,7 +1,19 @@
 // @flow
 import { Document, Team } from '../models';
 
-function present(document: Document, team: Team, context?: string) {
+type Action = {
+  type: string,
+  text: string,
+  name: string,
+  value: string,
+};
+
+function present(
+  document: Document,
+  team: Team,
+  context?: string,
+  actions?: Action[]
+) {
   // the context contains <b> tags around search terms, we convert them here
   // to the markdown format that slack expects to receive.
   const text = context
@@ -13,8 +25,10 @@ function present(document: Document, team: Team, context?: string) {
     title: document.title,
     title_link: `${team.url}${document.url}`,
     footer: document.collection.name,
+    callback_id: document.id,
     text,
     ts: document.getTimestamp(),
+    actions,
   };
 }