Add Epics menu

What does this MR do?

In this MR, we're refactoring the Epics menu. We're moving it from being a partial rendered menu to an object-driven menu, like all the menus in the project sidebar.

For that, we're just introducing a new object to store the menu info and removing the related info from the menu partial.

Screenshots or Screencasts (strongly suggested)

This is the refactored menu, which looks exactly like the older 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 #328504 (closed)