Dynamic subscription

[newearthmartin]

The goal of this PR is to be able to dynamically generate a list of subscriptions at the moment of submitting the Submission. It is following on this idea https://github.com/jazzband/django-newsletter/issues/359.

The Newsletter class has a new field subscription_generator_class. If this field is empty, it will continue behaving as usual. However, if the field is not empty, It is a class name that will be instantiated into a subscription generator object. The subscription generator will be in charge of generating the list of subscriptions at the moment of sending the message.

This is done by implementing the generate_subscriptions(submission) method. This method must return a list of Subscription objects to be used for sending the message. These dynamic Submission objects must be in-memory objects and should not be saved to the DB.

Existing Subscription objects in the DB for this Newsletter/Submission will still be used. At the time of submission, dynamically generated Subscriptions will be added to DB Subscription objects. Also, unsubscribed DB Subscriptions will be removed from the dynamically generated Subscriptions.

[codecov[bot]]

Codecov Report

Merging #367 (5c1d961) into master (c350966) will increase coverage by 0.04%. The diff coverage is 89.28%.

@@            Coverage Diff             @@
##           master     #367      +/-   ##
==========================================
+ Coverage   86.02%   86.06%   +0.04%     
==========================================
  Files          16       16              
  Lines        1302     1328      +26     
  Branches      137      140       +3     
==========================================
+ Hits         1120     1143      +23     
- Misses        135      137       +2     
- Partials       47       48       +1     

Impacted Files	Coverage Δ
newsletter/admin.py	81.74% <33.33%> (-0.63%)	:arrow_down:
newsletter/models.py	91.96% <96.00%> (+0.29%)	:arrow_up:

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[newearthmartin]

@dokterbob curious if you have any opinions about this PR