Improve event definition disable/enable toggle when editing an event definition

## What?

When editing an event definition, the placement and wording to disable/enable the event definition should be improved to match the intuitive nature of the `Status` column on the `Event Definitions` page (`/alerts/definitions`).

## Why?

When looking at the `Status` column on the `Event Definitions` page, it's extremely easy to determine the current status of the event definitions, and equally as easy to toggle their state.

In comparison, when editing an event definition you have to:
1. Look for the checkbox input in the `Condition` section (e.g. `Filter & Aggregation`).
2. Understand that the `Enable` checkbox and `Should this event definition be executed automatically?` correlate to the event definition being disabled/enabled. AFAIK event definitions can't be manually executed, which could make one wonder if this was the disabled/enabled toggle. Easy enough to figure out, but could be made more consistent.
4. Understand that `Enable scheduling` in the summary correlates to the event definition being disabled/enabled.

Work with Graylog enough and you'll figure this stuff out, but a new user might get a bit confused.

## Your Environment

* Graylog Version: `6.0.1`