Simplify CPT and taxonomy registration by using the class name also as a slug

[timiwahalahti]

We are doing some stupid extra work by defining the slug and class name individually when both are almost always the same.

In this PR, a change is made so that the class name gets used as a slug automatically. Slug is transformed to a lowercase version, and the underscore is changed to a dash for the file name.

This simplified the cpt & tax registration process.

[height]

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