Add empty pipeline drawer [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

In the editor section, adds a pipeline drawer behind a disabled feature flag. This is the first part of an effort to add a contextual drawer with helpful tips to build you pipeline. There is only the drawer itself, which can open and close all behind the disabled feature flag pipeline_editor_drawer.

Description	MR link
Implement the empty drawer in PA section	You are here 👈
Add the drawer content	!60957 (merged)
(Nice to have) remember collapse state in local storage	link to come
Rollout issue	#329806 (closed)

Screenshots (strongly suggested)

before	after (collapsed)	after (expanded)

Screen_Recording_2021-05-03_at_3.13.36_PM

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is behind a disabled FF.
• [-] 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 #296547 (closed)