Make the plugin useable without permission plugins

[GeorgH93]

There are many users who try to use the plugin without a permission plugin. Currently the only workaround for them is to use the permissions.yml file, however this file is not accessible with all server hosting providers.

Required functionality:

• [ ] option to define which features should be available without a permission plugin
• [ ] option to enable the fallback permissions
• [ ] option to set the default backpack size for op / non op users

Nice to have:

• [ ] detect if no permission plugin is installed an ask op user if they want to enable the fallback permission handling
• [ ] auto config option for fallback permissions, which automatically turns them on if no permission plugin is found