Fixes bug 1013322 - Generate the elasticsearch mapping from the master list of fields.

@peterbe r?

Note that this feature is turned off by default through configuration, using the ``use_mapping_file`` option in ``ElasticsearchCrashStorage``. I want to turn it on on stage first and make sure the new mapping way doesn't break indexing. After a week or two of successful indexing, we will turn it on on production. 
I can see a problem arising here when:
 * Elasticsearch is down, which would impact the entire app anyway
 * the ``socorro`` index is not created, in which case it should be fixed or Elasticsearch should be disabled in the configuration. And I think it's better to let that bubble up because it's the developer's fault for not setting it up correctly. 

The ``SuperSearch`` class does not throw any exception in the stack that is involved here, so it would only be errors caused by Elasticsearch that would happen. And afaik the processors are pretty resilient to those errors and will retry to index a crash regularly.