Links between Phoenix <=> Rails are configurable

[stepchud]

There are a few places where the Rails app links out to the Phoenix app, and vice-versa.
For example the Challenge Manager's Utility Menu has a link to manage Challenges which navigates to the Portal. This link needs to be a configurable environment option so that it works in localdev (localhost:4000), challenge-dev deployment env (https://challenge-dev.app.cloud.gov/), staging and production.
On the Phoenix side, the Challenge Manager's Dashboard links back to Rails.

Acceptance Criteria

• Utility Menu and Dashboard links to Phoenix app work on local dev machines and on the dev deployment server
• Phoenix Dashboard links to Rails features work on local dev machines and the dev deployment server