Fullscreen pages need the resource pack

• Changed: a fullscreen page no longer opens for a player without the resource pack. Before, it seated them anyway and cleared the blocks in front of them to show the page on a wall of maps. Now they are told they need the pack and it is offered to them again. Wall screens still work without it.
• Fixed: with resource-pack.mode: external and the pack merged into another plugin's (such as ItemsAdder's), players who loaded that pack were treated as having no pack, so the pointer lagged and the world was cut open round fullscreen pages.
• Fixed: the message for a page that cannot open now says "you" to the player it is about.