config: require owner only if no owner_account is set

[dgw]

Description

This shifts validation of the core.owner setting from the base StaticSection class to CoreSection, by unmarking it as required (removing default=NO_DEFAULT) and checking the values after loading and validating the section has succeeded via super().

Comes with its own tests to validate the new behavior (a habit we should continue trying to keep up).

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

Notes

Will need to undo #2112 once that's merged, and remove any other references to owner being required no matter what.

Would have loved to raise ConfigurationError instead of ValueError, but config imports config.core_section:

ImportError: cannot import name 'ConfigurationError' from partially initialized module 'sopel.config' (most
likely due to a circular import) (/mnt/c/Users/dgw/github/sopel/sopel/config/__init__.py)

[dgw]

We actually have to keep requiring core.owner be set to the owner's nickname, even if core.owner_account is being used for auth. Sopel itself uses core.owner as the recipient for important messages/notices such as when a new version is available, or if something goes wrong during a netsplit and the bot loses its main nick. (Some third-party plugins surely also do similar things.)

I'm reworking the docs again instead, on top of the just-merged #2112; too bad I didn't think of this until after merging that.