Provide way to clear total errors count in pipeline rules

When reviewing pipeline rules you can see the error column shows per second/total. After fixing a noisy error, it might be beneficial to reset this total counter if it has exploded into a large number:
0 errors/s (4316839 total)

## Expected Behavior
There should be a button to clear totals next to the UI element showing these totals.

## Current Behavior
The total counter appears to be permanent until Graylog is restarted, which may not be preferable in a production environment.

## Possible Solution
Allow a means to store the total in mongodb rather than memory / be clearable via UI button options. Either 'clear all' or clear-per-counter.

## Steps to Reproduce (for bugs)
1. Generate parsing errors in pipeline rules
2. Look sadly at your total error count

## Context
Useful for troubleshooting parse errors in pipeline rules, once you clear the thing causing the rule it is easier to randomly check in on it if the 'total errors' count is a zero.

## Your Environment
N/A

* Graylog Version: Graylog 3.0.0+db6cf59 on [hostname] (Oracle Corporation 1.8.0_191 on Linux 4.4.0-142-generic)
* Elasticsearch Version:
* MongoDB Version:
* Operating System:
* Browser version: