Adds ability to work on new Ci config file to pipeline editor [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Issue: #300059 (closed)

Adds a CTA button so that users can start working on their Ci config even without a repository or a root Ci config file. The user flow is as follow:

• User in a brand new project without repo goes to the editor section
• Sees the empty section screen.
• ADDED: There is now a button to get started with CI/CD
• ADDED: User clicks on the button and see the text editor
• ADDED: The validator at the top of the page will have a useful message when the editor is empty.
• MISSING FROM backend: Ability to commit those changes.

This change is behind a disabled feature flag pipeline_editor_empty_state_action. We hide the CTA button if it's disabled, so the users cannot see this functionality. Because the BE is not done yet, it's not fully functional (if you try to commit the changes, it will not work). We will turn the feature flag on once the backend implementation is done. No changelog required as well.

Screenshots (strongly suggested)

Before	After

Screen_Recording_2021-03-02_at_2.13.33_PM

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• 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