Revert "[#1894] Simplify conversion tests"

This reverts commit d7f2454be39654b457a70b61e8feb3f13f32334f.
Revert "[#1894] Update docs and example_idatasetform to reflect changes"

This reverts commit ef571877cd552fd6158104439175953edcb4c404.
Revert "[#1894] (Partially) Fix the error_summary for extras validation errors"

This reverts commit 355d91d95fe95da1a5e394f83a536fe2abbd0b6c.
Revert "[#1894] Prevent free extras to have the same key as a schema field"

This reverts commit 00bee37364a239332a025a92e3abc7295487cd29.
Revert "[#1894] Pass a list of keys in the schema to validators"

This reverts commit 98afbf66b432b0dfe0958e9acae0417e4f64b950.
Revert "[#1894] Move test files to proper place"

This reverts commit cfadddc1a775fbbcd839bd0b5764991555aeface.
Revert "[#1894] Fix convert_to_extras to allow use with normal extras"

This reverts commit 6532c1677d4a74fab9b984fc30e4ae63ed2a0e40.