Reapply "Add back GPL license header to iframe.scss"

This reverts commit 1d215d8c75.

src/css/iframe.scss

@@ -1,3 +1,20 @@
+/* 
+ * This file is part of EvenBetterSEQTA.
+ *
+ * EvenBetterSEQTA is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * EvenBetterSEQTA is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with EvenBetterSEQTA.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
 body {
   background: transparent;
 }