Use drf_yasg to generate swagger

[davcamer]

Fixes:

#1853
#1855
https://github.com/digitalocean/go-netbox/issues/26

drf_yasg provides more complete swagger output, allowing for generation of usable clients. As explained in drf_yasg's README, the django rest framework is based on coreapi, which doesn't include response schemas. drf_yasg instead approaches the swagger with openapi, allowing it to generate full response schemas.

Some custom work was needed to accommodate Netbox's custom field serializers, and to provide x-nullable attributes where appropriate. x-nullable is an extension to OpenAPI v2, but is understood by the golang swagger generator I was targeting. I have not tried it with other generators.

This also replaces the api documentation at /api/docs with similar documentation, also based off of the swagger UI. There is alternative documentation at /api/redoc/ included only because it is by default part of drf_yasg. That could easily be removed if it isn't desired.

One other note: I was not able to get old_requirements.txt to work as expected with more than a single line in it. It simply errors out when the pycrypto is not installed, because of the previous upgrade.

$ pip3 --version
pip 9.0.1 from /usr/local/lib/python3.6/site-packages (python 3.6)
$ pip3 uninstall -r old_requirements.txt -y
Uninstalling psycopg2-2.7.4:
  Successfully uninstalled psycopg2-2.7.4
Cannot uninstall requirement pycrypto, not installed

Based on what you've done with psycopg2, I've moved django-rest-swagger to the top of that file.

[davcamer]

@jeremystretch I believe this is now complete. I've worked through the problems with the generated client that people identified so far in the https://github.com/digitalocean/go-netbox repository.