Refactor sidebar menus link method

What does this MR do?

Most of the sidebar menus default to the first menu item link as the main menu link.

Instead of writing it in every menu, we can just move it to the base menu class and let the other menus inherit it. We'd just need to override those with different behavior.

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.