Improve rendering time for API docs

[jeremystretch]

This issue was split off from #2665.

Environment

• Python version: 3.5.2
• NetBox version: 2.5.7

Steps to Reproduce

1. View the API documentation at /api/docs/.

Expected Behavior

The documentation should be rendered within an acceptable amount of time.

Observed Behavior

The page takes a noticeably long time to load, even to the point of prompting the user to kill the script running to render the docs.

Per @axnsan12 on #2665:

This is a real issue, with a two-sided cause:

• due to some unfortunate design choices, drf-yasg is quite slow; on my machine the XHR request for the swagger document takes around 10 seconds, and there's not really much that can be done to improve this
• drf-yasg configures swagger-ui with a default defaultModelsExpandDepth of 3, which means the models section at the bottom is fully expanded; this is the main cause of the initial stuttering

You could mitigate the first point by either

a. caching the generated schema
b. pre-rendering the schema during packaging, deployment or app startup