feat(webpack): configure publicPath via ASSET_BASE_URL env var

[nytai]

SUMMARY

allows webpack publicPath to be configured via ASSET_BASE_URL env var. This should enabled static assets to be hosted on a seperate server/CDN instead of serving them via the python webserver.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TESTING INSTRUCTIONS

manual testing:

• webpack build with ASSET_BASE_URL=http://localhost:9000 set, results in urls in the webpack manifest containing the prefix http://localhost:9000.
• All asset requests in the browser load and are not to relative paths.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API