The set up for the Feature Flag-driven Startup css

What does this MR do?

Sets up the basis for serving different versions of the Startup CSS based on the environment variables like a feature flag, for example.

Startup CSS helper changes

• Introduces startup_css_helper.rb with #startup_css_filename helper method to encapsulate around various variants of the startup css, and uses it from _startup_css.haml.
• Adds full test coverage for all logic branches of startup_css_helper.rb.
• startup_css_helper logic can remain unchanged and be used for future feature flags involving changes to the startup css.

⚠ To test locally, please, enable the :sidebar_refactor feature flag for your local user. The flag is scoped to the user and is not global

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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 #322680 (closed)