Move custom stages form vuex to separate module"

What does this MR do?

Moves the custom stage form vuex code into a separate module

• Fix: ensures the default selected stage must be active (previously if the first stage was hidden it would still be selected)
• Replace done callbacks with promises
• Fixes the flicker of empty data when editing a custom stage

A follow up MR will clean up the specs !30830 (merged)

Screenshots

Added loader for the edit stages form

Before - no loader	After - with loader
Screen_Recording_2020-05-07_at_10.27.22_pm	Screen_Recording_2020-05-08_at_12.01.03_am

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 to #214893 (closed)