Add a warning when checking personal messages from a user public profile, as an admin

[ondrej]

@Canapin

he explained that 50% of his user went away from his forum.

You mean this right?

@meglio

One of our competitors has recently lost more than 50 active users just because there were a proof that mods can read private messaging threads! There were a real scandal - it was read by many users; it all ended up in users leaving the forum (some of them have registered with us).