Fix dropdown icon misalignment when using fomantic icon (go-gitea#23558)

There are still many dropdowns using fomantic icon. For example: new
issue with issue template.

Avoid polluting the fomantic styles.

Before:


![image](https://user-images.githubusercontent.com/2114189/226101609-fa64178d-fed0-4541-bea2-72a250cb74db.png)

After:


![image](https://user-images.githubusercontent.com/2114189/226101603-9507b275-fc5b-4a55-870d-db8c4486d80a.png)

Author: wxiaoguang

web_src/css/base.css

@@ -2544,10 +2544,10 @@ table th[data-sortt-desc] .svg {
   height: auto; /* reset the ".ui.dropdown > .dropdown.icon {height}", otherwise the icon would be too small */
 }
 
-.ui.selection.dropdown > .search.icon,
-.ui.selection.dropdown > .delete.icon,
-.ui.selection.dropdown > .dropdown.icon {
-  top: 0 !important;
+.ui.selection.dropdown > .svg.search.icon,
+.ui.selection.dropdown > .svg.delete.icon,
+.ui.selection.dropdown > .svg.dropdown.icon {
+  top: 0 !important; /* reset the ".ui.selection.dropdown > .xxx.icon {top}" if the icon is svg instead of the fomantic icon */
 }
 
 .ui.dropdown.no-text > .dropdown.icon {