Back-end allows moderators to use merge/move posts to private message, but front-end does not

The decision to not show “move to new PM” option for moderators was made explicitly in:

https://github.com/discourse/discourse/commit/dc2776bcddc07e69e5a4ab92526f6238f3a5ccd7

This is because the moderators do not have permission to see PMs and the relevant code will fail at: