Newsletter owner groups

[ptitloup]

I created this branch to add owner group(s) to newsletter. If newsletter have group(s), only users of this groups (and superuser) can manage it and his messages, subscription and submision... Newsletter groups are not required, it doesn't change anything if you don't want to use it. Please let me know if it contains mistakes. Regards

[coveralls]

Coverage decreased (-1.7%) to 86.895% when pulling aef8b1daf6ccfcf7393125525673d7d18ce1d208 on ptitloup:ptitloup/add_group_to_newsletter into 30c3ec3f3bea93b22c6640aed40ee2c8040a4f46 on dokterbob:master.

[coveralls]

Coverage decreased (-1.6%) to 87.028% when pulling 63648f5e80357b1cb91f54dcfe73e2752d26edc2 on ptitloup:ptitloup/add_group_to_newsletter into 30c3ec3f3bea93b22c6640aed40ee2c8040a4f46 on dokterbob:master.

[dokterbob]

Thanks for the contrib. This is quite the feature and I'm quite loaded with work right now.

Please bear with me that it might take up to several months for us to properly review this and get back to you. In the mean time, the PR will stay open so others are able to use it and (hereby) are also explicitly invited to give feedback!

Of particular interest to me is to assure that:

1. It doesn't break any existing functionality.
2. Doesn't decrease coverage.
3. Is fully documented.

[dokterbob]

@claudep Any chance you could have a look at this one, so @ptitloup doesn't have to wait so long?

[claudep]

Not sure if I'll find time, but for better chances, the patch should also be stripped from layout and convenience changes. @ptitloup, only change what is required for the new functionality. Then you can suggest formatting changes in a separate patch if you want.