Divide simplenews admin pages in 'content' and 'settings'

This issue is for documentation/discussion purpose. The patch is already committed.

The patch divides the Simplenews admin section over /admin/content and /admin/settings.
The content section now contains: Send issues | Draft | Newsletters | Subscriptions
The Settings sections contains: General | Newsletter | Subscription | Mail backend

I still doubt if the Subscriptions admin section should be moved from Content to the Admin Users section. This would open up the possibility for a separate subscription admin permission. But is it also a logical place to search for Subscription administration?

All admin/content/newsletter url's have changed to admin/content/simplenews just to be more strict on the use of name space.
A new permission for 'administer simplenews settings' has been added.