Push notification service for our Enterprise customers

Since Defguard is selfhosted we should provide two possibilities (this is for analysis how it should be done):

### Own selfhosted based push

This will require the admin/company to do all the work, obtain Google/Apple push certificates, etc..

### Our cloud based push notification service

This service should:
a. Be open source so that our clients should see how it's working
b. Be available only for Enterprise customers - since we need to host it and make it available
c. Should not be enabled by default - there should be a clear explanation that this service does not send any data, only "invokes" messages for the mobile app to be opened etc..