Dynamic solr fields, index groups and orgs

Our solr configuration is currently limited to datsets and the fields are indexed as strings (except for *_date fields). For many users this results in forking and maintaining their own version of the solr schema.

Let's add dynamic support for repeated fields, language fields in the correct language, numbers? other types? So that fewer people need to maintain a solr schema fork.

Let's also index datasets with a `class='dataset'` or similar so that we can add groups, organizations and other things we might want to search and facet on in the same index.