Lazy render diff line comment button

What does this MR do?

Changes how the diff line comment button gets rendered. Instead of always getting rendered into the DOM, it now is not rendered int he DOM by default. The button only gets added into the DOM once the user hovers over a diff line.

Once the user hovers over a diff line the button gets added into the DOM and show. On subsequent hovers, the button usesthe old method of show/hide instead of getting removed and then re-added into the DOM.

This cuts the costs of Vue having to initialize the icon component inside of the button on every diff line which for larger diffs could be into the thousands.

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
• 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

Closes #223077 (closed)