Add Content-Pack mapping feature

### Problem description

Expanding: Graylog2/graylog2-web-interface#1396

I create Content packs for Backup purposes and used it to Merge from 1.3 to 2.0 (new HW so no upgrade). (it was very convenient btw)
So my Problem is at the dashboards. After the import my Stream-based queries are all messed up, because the streams have new ids. Most of my Dashboard queries sourcing from some Streams, because my Team want to use the "Play"-Button to see the underlying data. And since i use Graylog mainly bcs. of the role-based access-control, i have to make Stream-Based queries.

It would be very nice if we can map the old stream ids to the new before the import is done. It could be implemented in some Step by Step import Interface.
### Steps to reproduce the problem
1. Export Dashboards in Content pack
2. Import Content pack on another Graylog-Machine
3. Fix all queries :(
### Environment
- Graylog Version: 1.3 and 2.0
- Elasticsearch Version: -
- MongoDB Version: -
- Operating System: -
- Browser version: -