Event Definition: Template Custom Field

## Expected Behavior
It should validate the syntax for a Template Custom Field.

## Current Behavior
A user can enter invalid syntax and save it, which prevents the event definition from being executed and causes error messages in the logs.

## Steps to Reproduce (for bugs)
1. Start Creating an event definition
2. On the wizard step `Fields`, add a template custom field with the syntax `${source. asdf}`
3. Save the event definition

## Context
The user might miss important alerts or events if the event definition is not executed.

## Your Environment
* Graylog Version: 6.1