Send submission to entire newsletter if subscriptions are empty

[sutyrin]

Hey,

Thanks for great app.

Caption for Submission.subscriptions says:

If you select none, the system will automatically find the subscribers for you.

I've stumbled upon this non-working and supposedly fixed, it works for me.

Please review, thanks!

[sutyrin]

hmm, I see that introducing custom related_name breaks things. Will repair tests.

[sutyrin]

Ok, wrote another test that seems to cover this case.

Initially, Submission.from_message, that is used extensively in the tests, seems to copy subscriptions from newsletter, which is not exactly replicates the case when submission is created from web with no subscriptions added explicitly.

[w3ichen]

I forked this repo and implemented this pull request for my own purposes.
If anyone is looking to use this fix, feel free to use my fork: https://github.com/MecSimCalc/django-newsletter