Store information about sent emails per issue

[thomasmassmann]

It would be nice to store the information about successfully sent emails and failed ones per issue. Currently the information is logged, but not available for further usage.

[MrTango]

As discussed, we should also use the info, for a retry send function. Meaning, when someone forgot to select some subscribers or groups whe could correct the selection and send the issue again. A ReceiversPostSendingFilter could make sure that no receiver get the email twice.

[MrTango]

realted to #95

[thomasmassmann]

We use the branch currently in a customer project and so far it is working great, customer is happy.

Current status:

• store recipients information with send status as annotation on newsletter issue once sending is completed (to avoid too much writing during send process)
• new IReceiversPostSendingFilter adapter filters recipient who already got the email, so issue will be sent only to failed and new recipients
• statistics page, allowing to reset statistics (for sending all emails again)
• reset statistics when issue is transformed into master

Some bugfixes and enhancements required for this to work:

• set cssutils log to error instead of warning
• IReceiversPostSendingFilter subscriber call is moved from plone user and group behavior to send method to filter all recipients and not just plone users
• IReceiversPostSendingFilter is also checking for INewsletterIssue adapters (which we use here to filter successfully sent emails)

[thomasmassmann]

Released as 5.0.0b1.