Fix height cache (#4909)

app/javascript/mastodon/actions/height_cache.js

@@ -0,0 +1,17 @@
+export const HEIGHT_CACHE_SET = 'HEIGHT_CACHE_SET';
+export const HEIGHT_CACHE_CLEAR = 'HEIGHT_CACHE_CLEAR';
+
+export function setHeight (key, id, height) {
+  return {
+    type: HEIGHT_CACHE_SET,
+    key,
+    id,
+    height,
+  };
+};
+
+export function clearHeight () {
+  return {
+    type: HEIGHT_CACHE_CLEAR,
+  };
+};