Fix MR review bar hiding part of the right sidebar

What does this MR do?

This MR fixes an issue where the MR review bar might be hiding part of the right sidebar. This MR uses a class added on the body element (similarly to how performance and system headers do it). The reason for using class on the body is because the sidebar and the diffs apps are different Vue apps.

We've done similar work for the MR changed files tree !59150 (merged)

Screenshots or Screencasts (strongly suggested)

before	after
revew-bar-hides-sidebar

How to setup and validate locally (strongly suggested)

1. Open any merge request and go to the "Changes" tab.
2. Create a new review comment on the diff ("start review" button, not the "add comment now")
3. you should now see the review bar at the bottom of the screen
4. make sure the window height is less than 780px (you can use responsive design tools in your browser)
5. you should see both copy buttons at the bottom of the right sidebar, not just one (see screencast above to see what bug to look for)

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Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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

Related to #327890 (closed)