Feature request : being able to use keywords in alerting system

When creating an alert / event with condition type filter & aggregation, the time period can only be numerical values in seconds / hours / minutes

## Expected Behavior
Being to use keyword search when configuring an event / alert

## Current Behavior
Functionality not implemented, one can only use numerical values in seconds / minutes / hours when configuring an event / alert/

## Possible Solution
Add a keyword field as in streams searches.

## Steps to Reproduce (for bugs)
Create an event / alert, choose filter & aggregation condition type. Try to configure a relative period of time : you can't

## Context
It would be interesting to be able to use keywords, as in stream searches. For instance, I am monitoring Java's garbage collector, and especially the full invocation cardinality : the value itself is not interesting but the number of full invocations within a period of time is. Which leads me to search for full invocation in the present day since midnight, which I can do in streams and not in alerts / events.

## Your Environment
One graylog node

* Graylog Version: 3.1.2-1
* Elasticsearch Version: 6.8.3
* MongoDB Version: 1:3.2.11-2+deb9u1
* Operating System: Debian 9.11
* Browser version: not relevant : last firefox