Add ability to setup system-wide URL redirects

[GUI]

For the developer.nrel.gov migration onto api.data.gov, we needed a way to setup HTTP redirects for some of our website content (we previously had some large binary downloads on that domain that we no longer wanted there with the transition to api.data.gov). In order to set this up, we needed some way to setup arbitrary HTTP redirects for agency domains.

This got addressed in NREL/api-umbrella-router#7 (I'm just catching up on adding some issues here for tracking in our api.data.gov milestones). With the current implementation, we have to set these up via YAML config on the servers (since I don't think they'll change very frequently), but we could eventually expose this functionality via the web admin if necessary.

And while I implemented this for developer.nrel.gov's migration we just had another group wanting a redirect setup last week, so hooray for timing and reuse!