Fix many WorldManager issues

[Yeregorix]

Affected API methods are : World#save, WorldManager#loadProperties, WorldManager#loadWorld, WorldManager#copyWorld and WorldManager#moveWorld. See the commit descriptions for more details.

I also added json indentation when writing templates so it's easier to edit as some server owners may want to edit generation settings.

~~One problem persists: renaming or copying from a vanilla world to a custom world does not work because custom worlds need a template and vanilla worlds don't have templates.~~