Updated scrutinizer configuration
Fix support for notifications about new friends.

Support for relationships did not make it into the Elgg 1.9 notifications system, so this reverts changes related to it, and adds a custom solution.
Added method that returns display name of the notification target.

Depending on the entity type, the return value is:
1. Entity name or title
2. Excerpt from the entity description
3. String "unknown"
Move all content type specific notification logic to their own views.
Adds support for group invitations.