Extract a re-used event name to the constants file

What does this MR do?

The Merge Requests Diffs app uses an event hub, and has an event name that's spread throughout the codebase as a plain, primitive string.

Naturally, this makes it hard to update all at once, without either some kind of programmatic sed/awk solution, or manually hunting down every instance.

This MR just pulls that event name to the shared constants file and - critically - sets the precedent to centralize event names there in the future.

Screenshots (strongly suggested)

Very N/A, this is purely a refactoring MR.

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