Add email choices to finders

Ticket: https://trello.com/c/DzJdcftL/431-checkbox-email-signup-for-other-finders-3

This pull request adds new filtering options to finder-api metadata files. This is used by finder-frontend to generate the checkboxes on email signup pages.

We have also added an extra field to the metadata in order to decide wether the choice should be preselected by default on page load. This will be used by finder frontend to display preselected values