Add a few group events to tests/sampleData.php
Add a common interface for Messages and GroupEvents
Update GroupEvent database columns

- Added `type` column
- Renamed `group` to `group_to` for consistency and to prevent conflicts with MySQL reserved keywords
Add QueryBuilder::isBefore() and QueryBuilder::isAfter() methods for timestamp comparison
Provide event type when storing GroupEvents
Add support for fetching both GroupEvents and Messages from the database
Show both messages and group events on discussions