Plugin architecture (#4861)

* wip

* Refactor, tasks, processors, routes loading

* Move Slack settings config to plugin

* Fix translations in plugins

* Move Slack auth to plugin

* test

* Move other slack-related files into plugin

* Forgot to save

* refactor

webpack.config.js

@@ -25,6 +25,7 @@ module.exports = {
         include: [
           path.join(__dirname, 'app'),
           path.join(__dirname, 'shared'),
+          path.join(__dirname, 'plugins'),
         ],
         options: {
           cacheDirectory: true