Allow schema to set elasticsearch type and index

Sometimes we have date fields. Sometimes we have fields that we want to
analyze (and therefore include in _all for keyword matching).
Don't expand values for date fields

Now that we support date facets with allowed values we want to not
expand the values into a value/label pair.

We'll move to having date facets as proper types at some point with
before and after text inputs, rather than piggy-backing on the
multi-select functionality.
Add date_of_occurrence to AAIB report schema

Uses a multi-select facet for now with every year of imported report data
included in the list. The oldest date of occurrence in the imported is
1958-02-06.
Improve regex used for segregating date criteria

Rather than only treating fields that end in `_date` as a date field,
match any field with `date` in it, either at the edge of the word or
surrounded by underscores, eg:

- date_of_occurrence
- opened_date
- some_date_thing
Merge pull request #41 from alphagov/add-date-filter-to-aaib-reports

Add date filter to aaib reports