Feed UI Improvements (#27356)

Various improvements related to feeds:

- Fix markdown rendering
- Increase font size from 13px to default 14px via `flex-item`
- Add style to hashes
- Move the timestamp to title line. I realize it's not optimal for
translation, we may need to change all these translations

Before:
<img width="768" alt="Screenshot 2023-09-29 at 22 52 58"
src="https://github.com/go-gitea/gitea/assets/115237/edda8b84-23cf-4a43-90ad-a892798f4e6c">

After:
<img width="781" alt="Screenshot 2023-09-29 at 22 58 09"
src="https://github.com/go-gitea/gitea/assets/115237/7097474d-efcf-4f22-a2ab-834a4e25c4e8">

web_src/css/shared/flex-list.css

@@ -69,7 +69,6 @@
 }
 
 .flex-item .flex-item-body {
-  font-size: 13px;
   display: flex;
   align-items: center;
   flex-wrap: wrap;