Resolve "Extended user centric tooltips"

What does this MR do?

It adds Userpopovers, so instead of just showing a tooltip we show now in depth information fetchec from the API. Main focus of this first iteration was adding the basic version and showing them mainly on the issues and MR page.

Loading State:

Note: Our current default avatar is an image and not a grey circle as in the specs.

Loaded User Popovers:

Implementation Plan

• Popover gitlab-ui component
• Prepare User Links
• Prepare User Links in Markdown rendering
• Global Event Handler
• Show Popover with Skeleton Loading
• Load and Cache User Info
• Render User Info
• Debounce Display
• Style Popovers
• Polish code
• Fix tests
• Add tests

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.

Closes #50157 (closed)