Support Slack Channels in Schemata and Popup #5

Might be nice to be able to quickly jump into the Slack and join a channel through the popup.

Steps:

• Add optional slack_channel attribute (+ description whether to include # or not 😅 ) to the JSON schema
• Show Join #channel invitation (with target=_blank leading to https://softwarecraftsmanship.slack.com/messages/${slack_channel}/ in popup if present
• (Optional): Provide slack_channel_id as to be able to deeplink into slack clients using deep links alas slack://channel?team=${TEAM_ID}&id=${CHANNEL_ID} (our ${TEAM_ID} is T0A7W9V7C).
  • Explain how to find out a channel id in README
  • Provide fallback in case slack:// is not supported (is that possible?)