Validate Custom Message Template

## Description
* Validate the custom message template in the `validate` method of the SlackEventNotificationConfig and add "Invalid Custom message template" error to the result, so that the field comes in red. Today, the error message is in the bottom is not tied to the field.
* remove validate method from `execute` https://github.com/Graylog2/graylog-plugin-integrations/pull/639#discussion_r524469627

Refer Improvements section:
https://github.com/Graylog2/graylog-plugin-integrations/issues/623

## Screen Shot

## Steps To Reproduce
1. Refer screenshot to see, how to input a invalid custom message template.

## Environment
- Graylog Version:
- Elasticsearch Version:
- MongoDB Version:
- Browser Version: