Plugin hooks (#1452)

* Refactor session and plugin code
* Add context to job tasks
* Show hooks in plugins page
* Refactor session management

graphql/documents/queries/plugins.graphql

@@ -10,6 +10,12 @@ query Plugins {
       name
       description
     }
+
+    hooks {
+      name
+      description
+      hooks
+    }
   }
 }