Make small updates to GitLab chat UI

What does this MR do and why?

Make small updates to GitLab chat UI

• No border bottom radius depending on user (matches UI in source code chat bot)
• Increase spacing between content and sources
• Update source wording

Changelog: changed

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Follow steps in https://docs.gitlab.com/ee/development/ai_features.html#test-ai-features-locally (I'm not actually sure which are necessary for this specific MR)
• Click Help at bottom of side nav in bottom left corner
• Click Ask GitLab chat

#411677 (closed)