Mv sidebar assignee from karma to jest

What does this MR do?

Was working on another MR that introduces Graphql to the component sidebar_assignees, to test effectively i needed to move the spec to jest.

Working through converting the spec I noticed a few of the specs could be refactored and getting them to a place where they worked would be very challenging, and feature specs were handling the use cases i removed (though not ideal). But through using the spec file to test the new graphql feature Ill be able to write better maintainable specs now.

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