Make global classes usable

[oskarkk]

This PR:

• changes the main yaml node in classes.yml from classes to classitems, to be consistent with arena config files (we don't have to worry about backwards compatibility, because classes.yml wasn't working at all before my previous PR, wasn't usable before this PR, and right now it's entirely undocumented anyway)
• adds new configuration option to arena config: general.useGlobalClasses, which defaults to false (so, global classes are opt-in)
• generates the default classes in classes.yml if the file is empty
• if general.useGlobalClasses is set to true, classitems node can be removed and no longer is automatically populated with the default classes. Nothing changes for classes that have useGlobalClasses set to false (the default). If there are classes in classitems and useGlobalClasses is set to true, players can use both the global classes and the arena classes, with arena classes overwriting global classes in case of name conflicts

Some code was refactored. There was code in two methods that parsed class configuration, and they both were trying to do the same thing. One was for classes.yml (it wasn't working at all), the other was for arena config. Now instead of them there's one good method used both for classes.yml and arena configs. That's why there's more lines removed than added even though I've added a feature.

To do in a separate PR: commands for editing global classes