Implement django-push-notifications over django-pnm

[agconti]

Because of ZeroPush's recent acquisition , django-pnm may no longer work after January 31st. If the service shuts down, we should switch to django-push-notifications instead.

fyi: @glemmaPaul, @vpatel85

[agconti]

@glemmaPaul @vpatel85 the service will shut down 😭 #128

[glemmaPaul]

Yeah I heard that too @agconti , this shows that all those services are just one piece of extra abstraction that only could fail.. 😢

[agconti]

In an effort to keep this project lean and minimally opinionated, I've decided against baking push notifications in. The hope is that a more general boilerplate will better serve the community's needs and will minimize unrelated code in their projects.