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">

---------

web_src/css/repo.css

@@ -231,6 +231,7 @@
   display: flex;
   justify-content: space-between;
   align-items: center;
+  margin-bottom: 5px;
 }
 
 @media (max-width: 767.68px) {
@@ -2777,7 +2778,6 @@ tbody.commit-list {
 #repo-topics .repo-topic {
   font-weight: var(--font-weight-normal);
   cursor: pointer;
-  margin: 2px;
 }
 
 #new-dependency-drop-list.ui.selection.dropdown {