add JSON media types and outputs

[gumaerc]

Pre-Flight checklist

• [ ] Testing
  • [ ] Code is tested
  • [ ] Changes have been manually tested

What are the relevant tickets?

Closes https://github.com/mitodl/ocw-www/issues/129

What's this PR do?

This PR adds the proper mediaType and output values to the Hugo configuration to make sure that JSON is exported where a layout is defined to do so.

How should this be manually tested?

• Clone ocw-hugo-themes
• Spin up the webpack dev server in ocw-hugo-themes locally by running npm run start:site:webpack
• Clone this repo on the cg/json-media-types branch
• Run hugo server --themesDir /path/to/ocw-hugo-themes/, replacing the path to ocw-hugo-themes with your own
• Pick an instructor in the source ocw-www data and visit your local dev site verifying that the instructor exists and data is returned, for example: http://localhost:1313/instructors/0a90e4c5-5f99-646e-cbfc-402250d8aba5/index.json