Improve Slack integration to allow notifying private, public, and group channels (fixes #8569)

[tomaszn]

This change enables users to set private, public, or group channels as the target for Slack notifications.

It achieves this by removing the slack_user_id intermediary field, which was filled with a response after querying the Slack API with the user's email, and which limited the notification targets to users' private Slack channels. Before removal, the value is copied into slack_username field, overwriting the email address, which is no longer needed (and no longer works).

Detailed explanations are in #8569.

Test results

• Unit tests pass.
• SLA breach notifications are delivered successfully when slack_username is set to a channel names, with and without a "#" prefix.
• Also success when a channel ID is used.

Documentation

No changes in the documentation are needed. However, I noticed that the Slack administration page looks now differently, so I updated the relevant screen shot in the "Notifications" section.

Checklist

This checklist is for your information.

• [x] Unit tests pass.
• [x] Your code is flake8 compliant.
• [x] Model changes must include the necessary migrations in the dojo/db_migrations folder.