tweaked data_records_by_district_and_reported_date view. #421

view wasn't including district id correctly.

minor comment update
handle messages separately on export page #421

modified data_records_valid_by_district_and_form view to only emit
records with a form.

Removed data_records_by_district_and_reported_date view using the
multi-purpose data_records view instead.

Changed default export start date to previous month since it makes
little sense as a default when exporting during early days of a month,
subtracting one month first.
added exclude_cols query param to csv/xml form exports. #421

For example inlude `exclude_cols=1,5` in your query parameters to
remove the first and fifth column of an export.