Different beta message for MHRA Finders

As we want to specify why the Finders are Beta, the default message
doesn't really work. This commit adds the option to pass it to the
content store along with the actual options for the 2 Finders.
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
Merge pull request #93 from alphagov/different-beta-message-for-mhra-finders

Different beta message for MHRA Finders
Remove email choices for most Finders

With these being live on Preview we realised the desired behaviour isn't
correct. Removing them for now as this is blocking deploys. This commit
also changes the call to be a fetch and returns an empty array if there
are no choices.
Merge pull request #94 from alphagov/remove-email-choices

Remove email choices for most Finders