Resolve "NoMethodError: undefined method `unsubscribe' for nil:NilClass"

What does this MR do?

Fixes #199034 (closed)

Sentry: https://sentry.gitlab.net/gitlab/gitlabcom/issues/1196373/

Checks for the presence of noteable before rendering the page for unsubscribing.

Currently, this page returns 500 if the entity to be unsubscribed from has already been deleted.

By checking for the presence of the entity before-hand and by rendering 404 if it does not exist, we can prevent 500 errors on this page.

Screenshots

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

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