Remove app instance from Config for error handler setting

[ahopkins]

This stems from a conversation with @prryplatypus and @ashleysommer re: the change to get the ErrorHandler functioning as expected in v21.9. This unfortunately introduced app as a property of the Config object.

This PR removes that, reverting back to the previous methodology, but still retaining the as expected ability to apply changes to the Config post-instantiation and carry through to the route handler that was missing in earlier v21.

TODO

• How to handle post-init change to Config where ErrorHandler is not default
• Value checking when setting fallback

[codeclimate]

Code Climate has analyzed commit 918b06f and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 86.6% (86% is the threshold).

This pull request will bring the total coverage in the repository to 86.8% (0.0% change).

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[ashleysommer]

Thanks for making this change

[prryplatypus]

LGTM except the DescriptorMeta in the Config class, because I don't understand what it does 😅

[ahopkins]

It stores an internal reference to descriptors so that when setting a value we determine if we need to execute the setter or fallback to the dict update.