VSA - Create value stream form UX improvements

What does this MR do?

Addresses some minor UX pain points for the extended VSA create form, ensures we:

• Scroll to a new stage when one is added
• Focus on the name input when a new stage is added
• Adds some minor transition animations

No changelog added since this is behind a feature flag

Screenshots (strongly suggested)

Animations

Screen_Recording_2021-03-05_at_8.15.27_pm

Default value streams

Before	After

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 #299408 (closed)