compose: Have at least 1 new line before and after a giphy gif.

Until now, a giphy gif was inserted in an inline manner, but now we insert
it as a block, with 0 blank lines before and after it.

web/src/giphy.js

@@ -112,6 +112,8 @@ async function renderGIPHYGrid(targetEl) {
                     compose_ui.insert_syntax_and_focus(
                         `[](${props.images.downsized_medium.url})`,
                         $textarea,
+                        "block",
+                        1,
                     );
                     hide_giphy_popover();
                 },