Centralize how the Diffs app attaches/detaches from the event hub

What does this MR do?

Moves general event hub subscriptions and unsubscriptions into specific handlers for that purpose.

Why

The number of events that the diffs app may subscribe to is theoretically unlimited.
Having each listener in the created/beforeDestroy methods is messy and could potentially obscure other important things happening in those lifecycle events.

It may be overkill to move the event hub interactions to another file entirely, but moving them to a separate method is a simple way to encapsulate potentially unbounded lines of code or modifications.

Notes

The app instrumentation also uses the event hub, but it needs to be bound beforeCreate, so it's kept separate.

Screenshots (strongly suggested)

N/A, just refactoring

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team