Resolve "Styling of user, project and group avatars"

What does this MR do?

Render a rectangle avatar for projects and groups on the following pages:

Projects

• Project list on dashboard
• Project detail page
• Project settings (Edit)
• Project list on user profile page (Overview tab)
• Project list on user profile page (Personal projects tab)
• Project list on user profile page (Contributed projects tab)
• Project list on user profile page (Contributed projects tab)
• Project list in admin area
• Projects dropdown in navbar

Groups

• Groups list on dashboard
• Group detail page
• Group settings (Edit)
• Groups list on user profile page
• Groups list in admin area
• Groups dropdown in navbar

Misc

• Fork project view (when clicking the „Fork“ button on a project’s details page)
• Autocomplete Dropdown in comment field on issue/MR page: typing /cc @wortschi will show the autocomplete dropdown.

Screenshots:

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9682

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/52734

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• 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.
• Security reports checked/validated by reviewer

Closes #52734 (closed)