Migrate "threaded conversations" module to community-managed app

[pospi]

It would be great to take threaded-comments and bring it into the Holochain org as was done for the file upload module that @RyanHow started. Note the rename from "comments" to "conversations" as I think this is a more generic name.

Would also like to discuss with @lorenjohnson & @nmitch17 / @forcize / @delroh to figure out what the best architecture for this module is. The work I was intending to do on it from here was:

• Implement a total ordering of comments (probably via linked list), such that comments have a pointer back to the previous comment under each base entry and can be read in order. Some creative ordering / adjustment logic will be needed in the link management in order to resolve ordering conflicts resulting from network partitions.
• Implement update capability:
  • add "last edited" field
  • implement reading of previous versions
  • implement author-based validation for update action (can only edit one's own posts)
  • implement time-based validation for edit action (can only edit within X minutes)
    • wire up to DNA configuration to make this configurable at deployment time
  • wire up DNA configuration to allow/disallow updates
• package final module for consumption per Holochain/Rust guidelines