API urls are hard coded to localhost

[elihschiff]

Describe the bug
Many urls that are returned from the api are hard coded as localhost instead of using the url of the website.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://yacs.cs.rpi.edu/api/v6/listings
2. View: data>0>relationships>course>links>related
3. See error

Expected behavior
The url should be the same as the url you are requesting from. So on localhost it should stay as localhost but on yacs.cs.rpi.edu it should be that

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: Linux
• Browser: Firefox
• Version: 73