Re-enabling notes/comments in Snippet view

What does this MR do?

A chore MR: after migrating the Snippet view rendering to Vue, we forgot to re-enable comments' handling on the view. This MR fixes it. THe severity is ~S2 only because the functionality is behind the feature flag (:snippets_vue), however, the flag is supposed to be enabled globally on gitlab.com. Hence the severity and priority for the MR.

Testing

1. While on master, enabled :snippets_vue feature flag through the rails console.
2. Go to a snippet and try to add a comment: it won't be shown as added before you reload the page. Also, markdown formatting won't work in the field.
3. Switch to dmishunov-snippet-vue-notes branch (the one for this MR)
4. Go to the same Snippet and try to do the same addition of a comment:

• the comment should instantly be shown as added
• markdown formatting while writing the comment should also work as intended

Screenshots

No screenshots: it's the functionality that was not working.

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