MAINT: Clean up kupfer.core.settings about kupfer HOT 2 OPEN

I did a POC to see how does it look if we change configparser by json: DIFF

Goods:

• JSON can handle bool and int types (configparser seems to only support string);
• Default null and false values can be omitted;
• We don't need 'convert' (dict) to config to save;

Bads:

• At migration user loose config or we need a migration tool;
• One more method are required to merge jsons (dict) configparser does it by default;

Also removed dead code (ExtendedSettings) since we don't use it anymore (even with my pull requesting to use it again).

from kupfer.

My POC had a bug for some reason isn't getting default of plugin value. Won't fix this until someone say that we will keep in this path.

from kupfer.