Make Saturn URL Configurable

[cricketsloan]

Currently the Saturn host is hardcoded around here (location may have changed by the time somebody gets to this issue): https://github.com/DataBiosphere/cgp-boardwalk/blob/feature/commons/spa/src/app/files/shared/files.dao.ts#L195

It should be configurable, so that no new build will be required if it changes.

Unfortunately, it's not trivial, especially if the UI is moving towards being deployed on s3, so we can't simply pass an environment variable to the Docker container, as the Docker container may go away. Instead the value would need to come from some configuration API that is surfaced by either Azul or cgp-dashboard.

┆Issue is synchronized with this JIRA Story
┆Project Name: Boardwalk-UI
┆Issue Number: BOARD-81