Add "visit link" route.

[DFurnes]

What's this PR do?

This pull request adds the ability to visit short-links – Bert's specialty!! This reads the Link record we created in #59, and either returns a 302 to the destination URL or throws a "not found" exception if we haven't made that short-link yet.

How should this be reviewed?

This should be a quick one! Just the route & the test! 🚥

Any background context you want to provide?

The next step will be saving some information about the request (destination, user-agent, yadda yadda) into our S3 storage bucket. I'll tackle that in a follow-up pull request.

Relevant tickets

References Pivotal #172799447.

Checklist

• [x] This PR has been added to the relevant Pivotal card.
• [x] Added appropriate feature/unit tests.

[weerd]

The next step will be saving some information about the request (destination, user-agent, yadda yadda) into our S3 storage bucket. I'll tackle that in a follow-up pull request.