Tweak repo topics bar (#25769)

Tweak repo topics bar (#25769)

Minor tweaks to repo topics:

- Use gap instead of margin to align "Manage Topics" when no topics
present
- Add margin to description instead

Before:

<img width="1232" alt="Screenshot 2023-07-08 at 13 08 15"
src="https://github.com/go-gitea/gitea/assets/115237/a5d3586c-6cbf-4b74-8137-11d91f2cbb45">
<img width="1233" alt="Screenshot 2023-07-08 at 13 08 05"
src="https://github.com/go-gitea/gitea/assets/115237/59b18d93-e4cb-4f2b-9bc2-d6aa63f93827">

After:

<img width="1232" alt="Screenshot 2023-07-08 at 13 08 42"
src="https://github.com/go-gitea/gitea/assets/115237/470d42ad-3f7e-40f9-b0a1-203b4af77eb9">
<img width="1231" alt="Screenshot 2023-07-08 at 13 08 32"
src="https://github.com/go-gitea/gitea/assets/115237/42d18048-748c-4a3f-ab89-3403866cef34">

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