Stable/0.3.x: Fix $ref usage and remove unreferenced models from output

[tuffnatty]

According to the spec, type cannot point to a custom type, which should be performed with $ref. Also, validation produced warnings about multiple unreferenced models (mostly Write...Serializer) which were previously generated. This patch removes them as well.

[codetalks-new]

Hi, I love this feature. I think this feature should be merged in.

[tuffnatty]

@banxi1988 Sadly, the stable/0.3.x branch is not being maintained. You can either use my fork, or try to backport this fix to to the master branch, or use drf-yasg which is better maintained.

[codetalks-new]

@tuffnatty Thank you very much. drf-yasg looks nice. I'll try it.