Allow users to opt out from tracking

[wmnnd]

The Contact schema should be extended with a do_not_track boolean field. When it is set, URL redirects for click/open tracking are disabled.

• [ ] Add do_not_track field
• [ ] Disable link tracking for contacts with do_not_track enabled
• [ ] ? Enable logging for do_not_track opt-in/opt-out
• [ ] Add checkbox to form for tracking opt-in/opt-out - automatically set checkbox based on DNT headers