Feature/ogc 1725 news subscribe for categories

[Tschuppi81]

Newsletter: Support for newsletter category definition and subscription

An organization can define newsletter categories. Subscribing users can select their categories of interest and will only receive newsletters that report on at least one of their subscribed categories.

TYPE: Feature LINK: ogc-1725

Checklist

• [x] I have performed a self-review of my code
• [x] I considered adding a reviewer
• [x] I have updated the PO files
• [x] I made changes/features for both org and town6
• [x] I have added tests for my changes/features