Fix spacing in issue navbar (#30238)

Create a new `issue-navbar` class specifically for this bar, previous
class used in many places and I thought I had them all removed, but not
this one.

Fixes: https://github.com/go-gitea/gitea/issues/30226

web_src/css/modules/navbar.css

@@ -140,3 +140,8 @@
 .secondary-nav {
   background: var(--color-secondary-nav-bg) !important; /* important because of .ui.secondary.menu */
 }
+
+.issue-navbar {
+  display: flex;
+  justify-content: space-between;
+}