Move posts to private conversation

[techAPJ (Arpit Jalan)]

@techAPJ

Note that our search library does not support searching for message via message ID, it’s already on my to-do list to implement this feature.

This is now done via https://github.com/discourse/discourse/commit/a121d4077172f8bcbb1a2b64477dd908db796fbf and https://github.com/discourse/discourse/commit/59e3eecfa6d0a5e5143e2705558e2b7ed0d6cd77,