Create: Missions

- Added config options for:
  * Select the first currency available for rerolling
  * Adjust the chances of dropping a Mechanical Exchanger when compleating a hard mission
  * Select free rerolls per week
  * Command to be ran when a mission is completed
- Fixed crash due to Exchanger direction when using some optimization mods
- Luckperms support for free rolls per week: you have to give the permission `missions.freerolls` and add a [Context](https://luckperms.net/wiki/Context), it's called "amount". For example, you can give a certain player 15 free rolls per week.