(WIP) PayPal subscribtions

[PetrDlouhy]

This is partially working implementation of PayPal subscriptions. I was able to manage subscription creation, getting state and cancelling. This is based subscription support drafted in #217.

Although I was able to successfully manage communication with PayPal, I have got stuck and I don't know how to continue at this point. The main problems are:

PayPal requires setting plan_id and having Plan created at the time of creating the Subscription. This is not required when using Subscription button (like django-paypal does). It brings in whole lot of problems - the price can't be changed per user, it requires implementer to set up the Plans on PayPal and maintain consistency with server settings (especially if django-payments are used with multiple providers).

Getting notification of the payments I don't know how to capture the recurring payments. With PayPal Subscription buttons PayPal sends notification about the payment. I didn't find how to do that with API. There also might be possibility to check the Subscriptions through the API and check their (possibly changed) expiration.

[codecov[bot]]

Codecov Report

Merging #274 (b8360f6) into master (10b749a) will decrease coverage by 1.33%. The diff coverage is 59.18%.

@@            Coverage Diff             @@
##           master     #274      +/-   ##
==========================================
- Coverage   79.92%   78.58%   -1.34%     
==========================================
  Files          28       28              
  Lines        1783     1868      +85     
  Branches      212      223      +11     
==========================================
+ Hits         1425     1468      +43     
- Misses        251      283      +32     
- Partials      107      117      +10     

Impacted Files	Coverage Δ
payments/paypal/__init__.py	75.41% <52.94%> (-8.67%)	:arrow_down:
payments/models.py	75.65% <71.42%> (-0.35%)	:arrow_down:
payments/core.py	87.69% <100.00%> (+0.39%)	:arrow_up:

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[PetrDlouhy]

Now I was at least able to override the plan price. Although it is still not optimal, as it bothers user with creation of the plans on PayPal.

[PetrDlouhy]

I have re-implemented this in django-plans-paypal which uses subscription button from django-paypal. I have no time nor motivation to finish this PR, but I am leaving it open for volunteers to finish this.