CF #33 - Display issue's content only for subscribers ; add tests

Note: as conftest from apps subscription and editorial are becoming
too similar, we make a single fixtures files for both app.
PEPization
Review of #28 - rename subscribers group
Add activate method on TrialSubscription and use it in registration ; add tests
Add missing test for TrialSubscription.close() method
Merge pull request #44 from briefmnews/#33_non_subscribers_dont_see_full_briefme

#33 non subscribers dont see full briefme + review of #28