Test Sites Endpoint

Create new /sites endpoint to return the test sites.

changed milestone to %NSHM Model and Tool Improvements

added In Progress label

assigned to @bclayton

mentioned in merge request !134 (merged)

mentioned in issue nshmp-apps#159 (closed)

added Needs Review label and removed In Progress label

mentioned in commit dba4b552

closed with merge request !134 (merged)

removed Needs Review label