Search UI - Issue scope results filter by confidentiality

What does this MR do?

Issue #238314 (closed)

Refs #249265 (closed)

• Implements the frontend changes for adding a confidential filter to the issues Search scope.
• Hidden behind a feature flag :search_filter_by_confidential (see #244923 (closed)) since the backend changes are not complete (they will be done in another MR)
• some fixes to match the designs
  • Add HR under the drop downs
  • Adjust width of both filter drop downs (state and confidential)

Screenshots

Confidential Drop Down

State Drop Down

Video

This is what the new filter looks like with the flag enabled. It does not currently work so it will be released behind the feature flag.

confidential feature flag enabled

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