Fix New issue button consistency

Fixes https://codeberg.org/forgejo/forgejo/issues/2613
Based on https://codeberg.org/forgejo/forgejo/issues/2613#issuecomment-1654709

- add new class `list-header-issues`;
- add rules that fix button position;
- use variable to keep the vertical offset constant;
- fix gap for edit button.

Co-authored-by: Gusted <gusted@noreply.codeberg.org>

web_src/css/repo/issue-list.css

@@ -16,6 +16,14 @@
   flex-wrap: wrap;
 }
 
+.issue-list-new.button {
+  margin-right: 0;
+}
+
+.list-header-issues {
+  min-height: var(--repo-header-issue-min-height);
+}
+
 @media (max-width: 767.98px) {
   .issue-list-navbar {
     order: 0;