Lazy load components (#3879)

* feat: Lazy-load routes

* feat: Lazy-load modals

* feat: Lazy-load columns

* refactor: Simplify Bundle API

* feat: Optimize bundles

* feat: Prevent flashing the waiting state

* feat: Preload commonly used bundles

* feat: Lazy load Compose reducers

* feat: Lazy load Notifications reducer

* refactor: Move all dynamic imports into one file

* fix: Minor bugs

* fix: Manually hydrate the lazy-loaded reducers

* refactor: Move all dynamic imports to async-components

* fix: Loading modal style

* refactor: Avoid converting the raw state for each lazy hydration

* refactor: Remove unused component

* refactor: Maintain modal name

* fix: Add as=script to preload link

* chore: Fix lint error

* fix(components/bundle): Check if timestamp is set when computing elapsed

* fix: Load compose reducers for the onboarding modal

app/javascript/mastodon/actions/bundles.js

@@ -0,0 +1,25 @@
+export const BUNDLE_FETCH_REQUEST = 'BUNDLE_FETCH_REQUEST';
+export const BUNDLE_FETCH_SUCCESS = 'BUNDLE_FETCH_SUCCESS';
+export const BUNDLE_FETCH_FAIL = 'BUNDLE_FETCH_FAIL';
+
+export function fetchBundleRequest(skipLoading) {
+  return {
+    type: BUNDLE_FETCH_REQUEST,
+    skipLoading,
+  };
+}
+
+export function fetchBundleSuccess(skipLoading) {
+  return {
+    type: BUNDLE_FETCH_SUCCESS,
+    skipLoading,
+  };
+}
+
+export function fetchBundleFail(error, skipLoading) {
+  return {
+    type: BUNDLE_FETCH_FAIL,
+    error,
+    skipLoading,
+  };
+}