Fix group lists visual

What does this MR do?

• Reset p bottom margin for group lists to fix vertical alignment (we do it for projects, events feed etc. Probably should extract into mixin one day)
• Remove double border for group lists to be consistent with project lists

Are there points in the code the reviewer needs to double check?

no

Why was this MR needed?

Because group lists inconsistent with others visually. And vertically asymmetrical in several places. And double border produces this ugly effect:

Screenshots (if relevant)

before	after

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?