[FIX] web: BS4, left-align buttons in modal footers

Modal footers now use flex to vertically center buttons. So our
left-align style overridde had to be adapted. Also, we sometimes
use footer buttons as non-direct children of the modal-footer, which
is not a supported BS4 case, so we had to adapt that too.

addons/web/static/src/scss/modal.scss

@@ -27,6 +27,13 @@
 
         .modal-footer {
             text-align: left;
+            justify-content: flex-start;
+
+            > div, > footer {
+                // TODO this rule should not be necessary if we used buttons
+                // as direct children of the modal-footer as normally required
+                flex: 1 1 auto;
+            }
         }
     }