closes#6584
- Frontend Changes:
- adds 'Apps' to Navigation Menu
- adds 'Slack' as nested page to Apps
- adds `apps.css`
- adds `slack-integration` model and uses `slack-settings` custom transform to parse JSON file
- adds validation for `slack` model
- adds fixtures and `slack/test` API endpoint to Mirage
- adds acceptance tests for `apps-test` and `slack-test`
- adds unit tests for `slack-settings` and `slack-integration`
- Backend Changes:
- adds API endpoint `slack/test` to send Test Notification
- adds default-values for slack model
- sends payload to slack:
- text: the url of the blogpost / test message
- icon_url: url to ghost logo
- username: Ghost
- adds `slack/index.js` to send webhook to slack if
- a new post is published (if slack webhook url is saved in settings)
- user clicks on 'Send Test Notification' in UI
- adds `slack.init()` to `server.index.js` to add event listener
- adds unit test for `slack/index`
