Add aircraft_category as email signup choice for AAIB
Add case_type as email signup choice for CMA
Add development_sector as email signup choice for DIFD
Add vessel_type as email signup choice for MAIB
Add railway_type as email signup choice for RAIB
Add default preselected values to email signup

In order for specific email signups to have different preselected values
add them to the metadata choices for each format. Thios can then be
used by finder-frontend to decide wether to preselect the choice.
Merge pull request #92 from alphagov/feature/add-email-choices-to-finders

Add email choices to finders