Resolve "Value Stream Analytics - Custom ordering of stages"

What does this MR do?

This MR introduces drag and drop manual sorting for value stream analytics stages.

This is behind the customizable_cycle_analytics feature flag.

Should only be merged after !27801 (merged)

Screenshots

Screen_Recording_2020-03-11_at_11.37.29

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

Related issue

#196698 (closed)

Todo

• Ensure that ordering is enabled as soon as a custom stage has been added, i.e. no need for refresh.