Update frontend build path to be more descriptive

[allisonking]

Closes #937

While working on https://github.com/ethyca/fidesctl-plus/pull/39, I found it would be more helpful if fidesctl was writing its frontend admin-ui to a folder that said it is the admin-ui. In addition, there didn't seem much a point to nesting it under fidesctl/api/build/static anymore, and I thought it would make more sense to have it under src/ui-build/static/admin to make it clearer what is going on. This will also let us better add on other frontends if we need to, since we know this one will be in the subfolder /admin. So for instance, the datamap build can also go in the same folder, but under /datamap.

Code Changes

• Renaming the constants in the backend
• Renaming the folder path in the package.json script
• Updating the gitignore

Steps to Confirm

• Everything should still work the same. Can delete any old frontend build you had, and run npm run prod-export again. Then running nox -s dev should allow you to see the latest built frontend at localhost:8080. You'll have the build in the src/ui-build folder instead of fidesctl/api/build/static now.

Pre-Merge Checklist

• All CI Pipelines Succeeded
• Documentation Updated:
  • documentation complete, or draft/outline provided (tag docs-team to complete/review on this branch)
  • documentation issue created (tag docs-team to complete issue separately)
• Issue Requirements are Met
• Relevant Follow-Up Issues Created
• Update CHANGELOG.md

Description Of Changes

Write some things here about the changes and any potential caveats

[LKCSmith]

👀 🎉