Provide ability to cancel searches

We need an ability to cancel searches.

## Expected Behavior

It's possible to cancel (stop) searches with a stop button in the Web frontend.

## Current Behavior

It isn't possible to cancel (stop) a search started by mistake. Sometimes a bad search (wrong parameters) leads to Graylog problems. In worst cases the whole ES crashes.

## Possible Solution

Implement stop button functionality in the Web frontend.

## Steps to Reproduce (for bugs)

1. Start a search with bad parameter (e.g. for a long time range)
2. Wait a long time for results or get a timeout.

## Context

Bad user experience after hitting a wrong search.

## Your Environment

* Graylog Version: 3.0.0
* Elasticsearch Version: 6.5.4
* MongoDB Version: 3.4.7
* Operating System: Red hat Version 7
* Browser version: Firefox 60.1.0esr