Start group sidebar refactor by adding new group panel object

What does this MR do?

In this MR we start the refactor of the group sidebar panel. We're doing so by just introducing a group panel object that will centralize (later) the creation of menus.

In this MR we're not refactoring any menu (yet), we're splitting the group sidebar view and moving the menus to the group_menus view. From there, we'll start refactoring each menu one by one.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are 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.
• This change is backwards compatible across updates, or this does not apply.

Related to #335484 (closed)