web: Add missing CSS selector escaping.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

web/tests/lib/compose_banner.js

@@ -8,7 +8,12 @@ exports.mock_banners = () => {
     // zjquery doesn't support `remove`, which is used when clearing the compose box.
     // TODO: improve how we test this so that we don't have to mock things like this.
     for (const classname of Object.values(compose_banner.CLASSNAMES)) {
-        $(`#compose_banners .${classname.replaceAll(" ", ".")}`).remove = () => {};
+        $(
+            `#compose_banners .${classname
+                .split(" ")
+                .map((classname) => CSS.escape(classname))
+                .join(".")}`,
+        ).remove = () => {};
     }
     $("#compose_banners .warning").remove = () => {};
     $("#compose_banners .error").remove = () => {};