Implement new active state for group sidebar context menu

What does this MR do?

In this MR, based on #322687 (closed), we pretend to change the behavior (behind a feature flag) of what happens when you click on the scope or context group menu:

Before, when you clicked on that menu, it'd redirect you to the group main page and would set the Group overview menu as active:

Now, the context menu is going to become a single menu that can be active. That way, when we click on it, instead of making Group overview as active, it will set itself as the active menu:

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.

Related to #322687 (closed)