Page break in reports

**Feature request**

Add the possibility to generate manual page breaks in reports.

## Expected Behavior
By manually inserting page breaks, the unsightly current reports become more readable again and the page breaks no longer occur within graphics.

## Current Behavior
Currently, page breaks are also performed in the middle of graphics. This makes the reports difficult to read.

## Possible Solution
Give the user the ability to manually add page breaks or adjust the rendering of the pages so that the page breaks happen automatically in the right places.

## Your Environment
* Graylog Version: 3.1.2+9e96b08
* Elasticsearch Version: 6.8.1
* MongoDB Version: 4.1
* Operating System: SLES 12 SP4
* Browser version: Firefox, Chrome,...