Changeset b3374f6 in OpenWorkouts-current for bin

(#52) - Make map screenshot generation async and non-blocker of the dashboard
and user profile pages:

• workout.map_screenshot is a property again, returns a string with the static path (suitable for use with request.static_url()) for the map screenshot file if exists, none otherwise

• added a couple of helpers to build the proper screenshot name and full path on the filesystem

• added a view to generate the map if needed, returning the full static url to the screenshot file in a json-encoded stream

• added a new js ow tool that looks for workouts that still don't have a map screenshot, calling the view to generate that screenshot and updating the map screenshot when the file is ready

• modified the dashboard and user profile templates, so a dummy/placeholder image is shown when no map screenshot is yet ready, plus setup and calls for the js tool that generates the maps.