Allow Users to Create and Save Predefined Aggregation Widgets

## What?

It would be very handy to save an aggregation widget that you can then reuse from the 'Add Aggregation Widget' sidebar similar to some of the built in options you currently see under the Predefined Aggregation section.

## Why?
For Example;
When running ad-hoc searches/or replaying a search from an event definition I find myself constantly having to create the same few widgets over and over again e.g. Data table for message count by process name or pie chart of messages broken down by event_type etc. It would be nice to create these once and then bring them in with a single click later.

## Your Environment

* Graylog Version: 5.2.7 (Cloud)