Update ResponseExportAPI with tags

[smhall97]

Background Tags are added to a response on the response details page (see example). Event admins are able to add these tags to group applications in a generic way, to aid processes such as filtering applications and/or desk rejections etc. See #786 for more details.

Dependencies Depends on the ResponseExport API in #846.

Requirements Include an option to add the associated tags (zero or more) in the ResponseExportAPI in responses/api.py.

Definition of done:

1. The downloaded PDF includes the response tags at the top of the PDF, above the applicant details etc.
2. There are tests for the new functionality.