Improve the "generate map screenshot" feature

[borja]

Right now, the javascript code loops over all the workouts needing a screenshot, calling asyncronously the map-shot view for those workouts. This means that we start N background processes running chrome on each one and making the screenshot, but those processes will block a thread on the processes running openworkouts, until the screenshot has been generated.

After trying this in several cases where more than a couple of screenshots were needed, it clearly showed that this code is not very efficient.

One idea for improving this code is that we could have a view, with the OpenWorkouts root model as the context, that accepts a list of uuids for workouts needed those screenshots. Then, we call the code that generates the screenshots passing the list of ids, so we can run chrome once and make all the screenshots, then close chrome.