Work on the `mattermost-plugin` — integration of SB into Mattermost as a product/plugin as a proof of concept.

To do:
* [ ] Bundle backend with node.js as part of plugin
  * Various options investigated, including [nexe](https://github.com/nexe/nexe) and [pkg](https://github.com/vercel/pkg) but neither of these will work great due to dynamically loading and resolving of modules in the node.js sandbox implementation.
  * Most straight-forward option is to simply bundle the `node` binary per platform + a trimmed down `node_modules` modeled how `npx` does this. Once per platform.
* [ ] Fix CSS styling issues
* [ ] Store pages in DB instead of text files (general SB feature, not MM specific)
* [ ] Switch over SB plugin to use MM database (MySQL, Postgres) as backing store rather than SQLite
* [ ] Freeze plug configuration (don’t allow anybody or at most admins) to update plugs for security reasons. We may simply remove the `PLUGS` page.
  * What about `SETTINGS`?
      * Easy option: disable, don’t use
      * Fancier option: make them user specific with a layer on top of the FS
  * What about `SECRETS`?

To deliberate on:
* Consider page locking mechanisms, or re-implement real-time collaboration (would require introducing web sockets again and OT) — big project.
* Consider page revision options
* Scope of spaces, tied to:
  * Personal (default SB PKMS use case, no permission, collaboration issues)
  * Channel (old Boards model)
  * Team
  * Server