WebHost, Core: Move item and location descriptions to `WebWorld` responsibilities.

[ThePhar]

What is this fixing or adding?

Moving these changes from World to WebWorld to remove responsibility of these from core to WebHost.

~~Also includes some optimizations.~~

How was this tested?

Migrated tests and ran tests. Also ensured functionality remained the same.

If this makes graphical changes, please attach screenshots.

[el-u]

The implementation is flawed, it tries to control what should be a ClassVar using @property getters and setters (which affect an instance attribute).

Debugging reveals that

• DarkSouls3World.web.item_descriptions contains only "Cinders"
• DarkSouls3World.web._item_descriptions contains only "Everything"

That is because the initial assignment goes directly to the type and creates item_descriptions there (overwriting the property with a plain dict), while the property setter ended up never being called and therefore _item_descriptions was never modified. You'll have to rethink that whole design. (Using @classmethod @property isn't a viable alternative because that was only supported in a narrow interval of Python versions and has since been forbidden again.)

[ThePhar]

Updated PR and fixed merge conflicts.