Implement local filtering by value rather than using query params to filter API-side.

This greatly increases the speed of an export, as it seems that filtering API-side by status would cause API responses to take up to 4x as long.

It's more efficient to fetch every for the identifiers given and then filter by value locally - this also allows for more complex filtering that is not supported by the API.

Additionally, return status in export when querying by student_institution or student_academic_plan, as this is what is documented in the readme. Also ensure that our standard non-card fields (full_name, forenames, surname, status) are always written as export headers.

Closes #24 (closed)