Support sorting issues using `relative_position`

What does this MR do?

Adds a "Manual" sorting option for project and group Issue lists. This sorts by the relative_position column on the issues. Positioning is global per GitLab instance.

Descending sorting is currently not supported. Drag/drop sorting will be added, so for now only support an ascending sort order.

100 issues are shown (rather than 20) when sorting by relative_position

Note: Additional frontend work will be done in a separate MR. This MR should not be merged until that work is done and ready to be merged.

This is a backport of the core CE changes, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28566

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance 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

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 #9121 (moved)