Add more options to customize notifications on Slack

[andergmartins]

It would be useful to allow customising the Slack notifications using the UI, not only programatically (#416).

Maybe it would make sense to have specific fields for the Slack message in the Notification Workflow form, since the format is very different from Email messages. Something like this:

A client suggested using the subject line in the Slack messages, but it can’t differentiate among the different status changes. And it is harder to add links in the subject line (wasn’t designed for that). He would have to set a pattern for links (like http://www.publishpress.com|Visit), or a shortcode. The user is on charge to decide where he wants the notification. In the email or Slack. So links in the subject line won’t make sense for emails. (edited)

Tabs could be added for each notification channel: Email, Slack. So the specific fields will be housed inside each tab.

Some additional field could be added for the Slack messages, allowing to have specific settings for each workflow (global/default settings are recommended as well):

• Slack channel (global/default > workflow > user)
• Message theme: full or clean
• Message text (with format options)
• Attachments (add the post as attachment?), a specific text? Color, etc...
• Action buttons?
• Profile image and username

We can basically allow to set more options:

• https://api.slack.com/block-kit
• https://api.slack.com/docs/message-formatting
• https://api.slack.com/docs/message-attachments