Enable sidebar refactor ff [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

In this MR we're enabling by default the feature flag sidebar_refactor. This change will enable all the recent changes we've applied to the left sidebar.

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 properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• 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.