Allow threads to be merged

Users may create threads on the same topic or issue, splitting discussion. Currently moderators would need to lock one thread, direct users to the other, and possibly forward each message from the duplicate thread to the new one.

Implement an automated way to merge threads, as exists on most web forum software. You should be able to click a merge button on a thread, then select which thread to merge it with. Ideally messages would be merged seamlessly into the existing thread, but even a more rudimentary implementation would be helpful. Confirmations should be included to help prevent merging threads accidentally or incorrectly.

Somewhat related request: https://feedback.fluxer.com/p/1022/ability-to-move-messages