Support for JS-based config in tubular

[davidjoy]

https://github.com/edx/tubular needs to be updated to support JS-based configuration of MFEs.

This should look like:

• Updating the frontend_build.py and frontend_utils.py scripts to use JS-based configuration if it exists, rather than our existing YML-based config files.
• Ensure that the existing mechanism of passing in env_vars into FrontendBuilder.build_app continues to work, while also allowing that method to alternately copy over a JS config file if it exists.

[jsnwesson]

Just wanted to bring up that I will be aiming to work on this ticket in the upcoming weeks!