Filtering content packs does not reset pagination

## Expected Behavior
After filtering the content packs page, I expect to see the first page.

## Current Behavior
Filtering while browsing a page that is not the first one may display an empty list:


At first I thought the search did not return results, but manually going to the first page while the filter is applied shows the right results. I guess we need to reset the page number when the filter changes.

## Steps to Reproduce (for bugs)
1. Add 11 content packs
2. Go to the 2nd page
3. Do a search
4. There are no visible results

## Your Environment
* Graylog Version: Graylog 3.0.0-beta.4-SNAPSHOT (87afab1)