Rename Redactor classes to ReferenceRedactor

What does this MR do?

Follow-up MR from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29861#note_185397090. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29861 adds a InlineMetricsRedactorFilter class, which makes the naming of the RedactorFilter and Redactor classes more broad than they need to be. This MR renames them to ReferenceRedactorFilter and ReferenceRedactor - more appropriately corresponding to the ReferenceFilter classes.

This has no user-facing or logic changes, so it probably doesn't need a changelog.

Does this MR meet the acceptance criteria?

Conformity

• [n/a] Changelog entry
• [n/a] 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.
• Tested in all supported browsers

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