Create new value stream modal form

What does this MR do?

• Adds a modal form for creating a new value stream
• Display a success toast message when clicking submit

Notes:

• Requires the value_stream_analytics_create_multiple_value_streams feature flag
• This is blocked by !35822 (merged)
• A follow up MR will add the POST request to persist the new stream

Screenshots

Screen_Recording_2020-07-09_at_1.10.28_pm

Testing

1. Enable value_stream_analytics_create_multiple_value_streams
2. Browse to group VSA, for example /groups/gitlab-org/-/analytics/value_stream_analytics
3. The Create new value stream button should appear next to the title
4. Clicking Create new value stream will display a modal
5. Submitting the modal will display a successful toast message

NOTE: The api request to create the value stream is not included in this MR, so the stream will not be persisted.

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 #221202 (closed)