Refactored the left panel toggle

What does this MR do?

Slightly refactors the left-side panel's collapse/open toggle. Some things to note in this MR:

• instead of two chevron icons, we use just one. We simply flip it in the collapsed state
• instead of raw styles we use utility classes and the SCSS mixins
• we disable Startup CSS when the Feature Flag (:sidebar_refactor) is enabled. This is to not be confused by styles coming from the Startup CSS stylesheets which are not updated with the changes introduced by the sidebar's refactoring. Since the flag will be scoped to particular people, we don't need to care about the FOUC.

Screenshots (strongly suggested)

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)