Show note/warning when adding a block that makes a previously unlisted video/series/playlist listed

[LukasKalbertodt]

Whether a video/series/playlist is findable via search depends on its ACLs of course, but also on the "listed" status. The latter is derived from whether the item appears on a non-user realm. Not all moderators might be aware that adding a video/series/playlist block has that effect. So it might be useful to add a little note when adding (or editing) a block that makes an item "listed" which previously wasn't.