Chat notification label support

What does this MR do?

Adds a label filter option to chat notification services.

This enables users to specify labels that they want to receive chat notifications about. This feature will be available for the Mattermost notifications and the Slack notifications integration.

The idea is that ~"group::ecosystem" can dogfood the slack integration by receiving updates about issues and merge requests that are labeled ~"group::ecosystem".

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done