Adding UserDownloadToken to store everything needed to generate a download link for a user in a tamper-resistent token
Update the GenerateForm worker to use the UserDownloadToken, and pass that to the mailer

Because using the URL helpers in a worker is a PiTA
It is much easier to generate the download URL from the mailer than the worker
Rename GenerateForm to GenerateAbsenteeForm

More specificity = more good!
Rename PdfHelper to AbsenteeFormHelper

More specificity = more good!
Refactor GenerateAbsenteeForm & AbsenteeFormHelper:

* Move all of the S3 stuff in to the helper, and also create an abstract helper for most of it because the S3 specifics will not change between forms
* Rename form_for_user and form_path_for_user. Those names were terrible.
Make the download link from an email work