Replace statistics pages with views

The goal is to get rid of all the code that builds statistics pages for newsletters and replace them with views as much as possible.
This issue will provide an overview of all related issues (bugs, features, task, ...)

The new views integration can be found at:

• admin/reports/simplenews/statistics
• on every node at node/%/simplenews_statistics

Bugs

• #1419684: Tabs opens and clicks don't work correctly

Tasks and new features

• #1321016: add download function for data per newsletter send - solution found