Merge branch 'fix/background-modal' into 'dev'

fix: modal background to fixed

See merge request web-et-numerique/pamn_plateforme-des-acteurs-de-la-mediation-numerique/pamn_client!105

src/styles.scss

@@ -191,7 +191,7 @@ button {
   width: 100%;
   height: 100%;
   z-index: $modal-z-index;
-  position: absolute;
+  position: fixed;
   content: '';
   top: 0;
   left: 0;