The /usepack command will now store the pack that a player has selected even after he logs out. It will re-apply the pack once the player logs in and use that pack on any server/world that doesn't have a special pack applied. To remove the stored pack just reset your pack to the empty one by selecting the name of the empty pack (like /usepack emptypack if the empty option is set to the emptypack pack).
Added a third parameter to the /usepack command to only give the player a pack temporary (until it is next changed or he logs out): /usepack <packname> [<playername>] [<temporary>]. temporary can be set to true or false (default behaviour). Permission to set temporary packs: forceresourcepacks.command.usepack.temporary