Edit custom cycle analytics stages

What does this MR do?

Adds support for editing custom cycle analytics stages for #13076 (closed)

• Separates the isAddingCustomStage state item into isCreatingCustomStage and isEditingCustomStage to better reflect usage
• Move form title and button title to computed properties to support editing / creating
• Replaces selectedStageId state with selectedStage
  • Replaces the SET_STAGE_ID action with SET_SELECTED_STAGE
  • Removes the currentStage getter
• Adds vuex actions to persist stage edits
• Re-fetch cycle analytics data on successful edit

Screenshots

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

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

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