PC file downloads: Retrieve via server

[jvwong]

Description of new feature

The file downloads module design introduces an issue where the PC_URL configuration needed to be identified on app boot when static assets were built before hand (i.e. in a Docker image).

To resolve this, retrieve the PC file downloads proxied via the server, rather than directly from the browser.

Motivation for new feature

• ease configuration and Docker image build
• security: queries could be done in the backend of the same app/domain, and because we might add PC2 API access JWT tokens in the future to prevent DDOS/abuse attacks