Remove lodash from core (#2827)

* Remove `lodash-es` dependency

* Replace `escapeRegExp` with home-made util

* Replace `throttle` with `throttle-debounce` library

* Use native browser methods for `deepFlatten`

We need a polyfill for iOS 11 and below. I think using a native method with this polyfill is better than having our own function instead, even if the bundle size is ~150B more.

* Save a few bytes in `escapeRegExp`

* Fix typo in comment

* Undo import re-organisation

* Use spread instead of slice

* Use smaller Array.flat polyfill from MDN

* Export new utils in `compat.js`

js/src/forum/components/DiscussionListItem.js

@@ -15,8 +15,8 @@ import slidable from '../utils/slidable';
 import extractText from '../../common/utils/extractText';
 import classList from '../../common/utils/classList';
 import DiscussionPage from './DiscussionPage';
+import escapeRegExp from '../../common/utils/escapeRegExp';
 
-import { escapeRegExp } from 'lodash-es';
 /**
  * The `DiscussionListItem` component shows a single discussion in the
  * discussion list.