FIx typo in Job help text
add request context for unsubscribe view

Adding the request context to the unsubscribe view allows for a more
versatile unsubscribe template
add a public view mechanism for jobs

Some newsletters should be viewable by the public. A new `public_slug`
field on the Job model describes which jobs should be viewable, and by
what URL.

Additionally, new public_url newsletter template parameter allows
sharing links in the newsletter to point to the public location instead
of the mail-specific url.