Add Safari like browser option

[goinnn]

I think the WebPushDevice browser field should be removed. But in the meantime, I think we should add the Safari type to the options, as mentioned in this comment:

#673 (comment)

To remove the browser field, registration_id should be the full URL instead of only the token:

https://github.com/jazzband/django-push-notifications/blob/master/push_notifications/webpush.py#L10

But that will be a big change.

This PR fixes: The REST API is failing if we try to register a Safari WebPushDevice

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.76%. Comparing base (44977b1) to head (66cc924).
Report is 59 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #776      +/-   ##
==========================================
+ Coverage   70.39%   72.76%   +2.37%     
==========================================
  Files          27       31       +4     
  Lines        1101     1252     +151     
  Branches      180      207      +27     
==========================================
+ Hits          775      911     +136     
- Misses        288      298      +10     
- Partials       38       43       +5     

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