Feature: Frontend Subscribe Form

[david-smejkal]

Goal

Implement a way for Birdsong to accept mailing list (campaign) subscriptions from frontend.

Implemented the following:

• create subscribe form template tag (birdsong_subscribe_form)
• implement /subscribe endpoint that will accept raw form submission requests
• implement /subscribe_api endpoint that will accept (ajax) json form submission requests
• prevent duplicate contact creation
• implement robust error handling to:
  • guard against xss and sql injections
  • utilize Django's and browser's built-in email field validation
• implement token /activate endpoint
  • send subscription activation urls using BIRDSONG_BACKEND
• write unit tests
• write a Subscribe form README section to:
  • demonstrate how to use the new birdsong_subscribe_form template tag
  • list and explain various new subscribe form related settings

Decided not to implement the following (yet):

✘ provide pre / post validation / form submission hooks (both for python and js)
✘ implement a Turing test (let devs choose between reCaptcha and hCaptcha)
✘ implement a 3rd party free opensource API email verification

NOTE:

• existing birdsong contacts will be automatically activated upon db migration
• changed birdsong_contact email field to be unique to prevent duplicate subscriptions
  • bumped up version of Birdsong to v1.2.0 (since this is potentially a db breaking change)
• redefined BaseEmailBackend as an abstract class with two abstract methods (send_campaign() and send_mail())
  • implemented send_mail() in smtp.py birdsong backend
  • bumped up version of Birdsong to v1.2.0 (since this is potentially a breaking change for custom backends)