Extract environments CSS from application bundle

What does this MR do?

This MR moves the environments style to its own bundle, out of the main application one following the steps described in the How to (which also suggest the utilisation of CSS variables instead of SCSS ones).

Screenshots

The screenshots provided present some cases that are mostly showing how removing the environments.scss files altogether seem not to affect the page so much. It hints to the fact the probably most of the environments.scss can be erased in a future iteration.

CE

Note: only difference with or without CSS is the banner

No CSS	CSS

EE

Note: looks the same with or without CSS

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