Customizable cycle analytics - Persist custom stages

What does this MR do?

⚠ Requires: !18514 (merged)

As part of #13076 (closed) we need to persist custom cycle analytics stages and display the custom stages in the stage table

• Adds vuex actions and mutations to persist custom cycle analytics stages
• Adds feature specs for creating a custom stage
• Fetches the updated stages after successfully creating a stage

Screenshots

Before adding a stage	After adding a stage

Demo

Screen_Recording_2019-11-14_at_9.41.35_pm

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