Customizable cycle analytics - Custom stage form ui

What does this MR do?

isComplete = false	isComplete = true

Label event selected	Start label selected	Start event changed

⚠️ Blocked by:

• !15122 (merged)

Implements the custom stage for UI for #13076 (closed)

• Start event
  • Filter the initial request for can_be_start_event
• Stop event
  • disabled until a start event is selected
  • displays an error and clears the stop event if a new start event is chosen that is not compatible
• Optional field for extra parameter
  • Currently label fields are the only fields that will require an extra parameter
  • Uses Single label selector
• Add / Save stage button
  • Emit a submit event
• Cancel button
  • Clear the values and close the form
• Follow up added to migrate this to vuex

CE Backport

CE Backport

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