Issue sidebar and misc css fixes (#15524)

- Replace remaining font icons with SVG in issue sidebar
- Rework issue due date display
- Realign avatar in timeline
- Fix font size in repo search and code explore
- Consolidate active button styles
- Fix loading form on arc-green
- Align time tracker buttons vertically

Fixes: https://github.com/go-gitea/gitea/issues/15896

web_src/less/_repository.less

@@ -815,7 +815,7 @@
         .avatar.image img {
           width: 20px;
           height: 20px;
-          margin-right: .5rem;
+          margin: 0 .25rem;
         }
 
         &:first-child:not(.commit) {