Adds campaign filtering

Fixes #11 

Adds filtering to campaign listings in individual digest messages. Filtering is based on:
- Remove campaigns that have a report back
- Remove campaigns that don't have a campaign image
- Remove entries that are not campaigns (invalid entries due to the migration from the old world to the new world)
- Only include regular campaigns, not SMS
- Remove inactive campaigns

- Sort by signup date, oldest first
- Limit the number of campaigns to 5 maximum