WIP: Adding support for events to send messages to multiple Mattermost channels

What does this MR do?

This MR allows the Channel handle fields in the Mattermost notifications page to support multiple Mattermost channels (with their names separated by commas or semicolons). When the relevant event is triggered, the message will be sent to all the specified channels.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Security

This MR does not contain changes to processing or storing of credentials or tokens, authorization and authentication methods, or other items described in the security review guidelines.