Add a feature to scrape Slack Channel Canvases

[davidfsol5]

Feature request:
As BlockScience, I want to store information stored in Slack Channel Canvases within my organizational know so that I can preserve their contents as historical record independently of their Channels and Slack as a platform.

Acceptance criteria:

• Canvases of both public and private channels must be preserved, but access to private channel information must be limited to the members of the private channel. This includes both direct access and indirect access, e.g. (but not limited to), access leakage through KOIs natural language search and generative capabilities.
• Preserved Slack Canvases must be stored periodically (snapshots) in such a way that different snapshots of the same Canvas can be recalled.