Explicitly define when to first trigger the search when creating event definitions

## What?

Provide a mechanism for manually setting the search time when creating event definitions. It doesn't look like there's currently any method for defining _when_ Graylog should first begin searching for an event.

## Why?

I imagine this is probably a somewhat niche desire when most event definitions are set to "search within every x seconds" or "y minutes" but I have some searches that, for operational reasons, can only be run every 24 hours. I monitor logs for some embedded readers on the international space station that are only downlinked from orbit 1x/daily. Ideally I'd like the event definition to let me explicitly define _when_ to first begin searching through logs for a defined event. This way I can ensure we get email alerts in a timely manner and know that updating/modifying the event definition doesn't change when the email alert is generated. See [my Graylog community post](https://community.graylog.org/t/determing-when-an-event-search-will-be-triggered/20894) for more details

## Your Environment

* Graylog Version: 3.2.6+e255fcc (from graylog-3.2-repository.noarch) -- Also confirmed this feature is still not available in 4.0.11
* Elasticsearch Version: 6.8.18
* MongoDB Version: v4.0.26
* Operating System: CentOS 7