Fixes the sliding problem with startup.css rendering

What does this MR do?

Fixes the sliding animation with startup.css by adding a body class when the page is really rendered (added a timeout of 1000ms).

With this we might also be able to fix the problem on mobile that sidebar is always shown and then hidden. Also we should see if we really should animate the content-wrapper as it is a lot of content that we are moving which is perhaps not that performant

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