Tidy up Activity feed

What does this MR do and why?

Tidy's up the Activity feed by:

• Changed the User avatar in Vue implementation to 24px (from 32)
• Move the username inline (both HAML and Vue)
• Changes the font size of the timestamp to small (both HAML and Vue)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable FF Feature.enable(:profile_tabs_vue)" | rails c
2. Check http://gdk.test:3000/root (Username should be inline now)
3. Disable FF Feature.disable(:profile_tabs_vue)" | rails c
4. Check http://gdk.test:3000/root (Timestamp should using small font)
5. Check http://gdk.test:3000/flightjs/Flight/activity

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to Update profile page layout (!134569 - closed), ✂️ UX Paper Cuts 16.6 → Activity (#425468 - closed)