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/common/compat.js

@@ -12,7 +12,9 @@ import Drawer from './utils/Drawer';
 import anchorScroll from './utils/anchorScroll';
 import RequestError from './utils/RequestError';
 import abbreviateNumber from './utils/abbreviateNumber';
+import escapeRegExp from './utils/escapeRegExp';
 import * as string from './utils/string';
+import * as ThrottleDebounce from './utils/throttleDebounce';
 import Stream from './utils/Stream';
 import SubtreeRetainer from './utils/SubtreeRetainer';
 import setRouteWithForcedRefresh from './utils/setRouteWithForcedRefresh';
@@ -91,6 +93,7 @@ export default {
   'utils/abbreviateNumber': abbreviateNumber,
   'utils/string': string,
   'utils/SubtreeRetainer': SubtreeRetainer,
+  'utils/escapeRegExp': escapeRegExp,
   'utils/extract': extract,
   'utils/ScrollListener': ScrollListener,
   'utils/stringToColor': stringToColor,
@@ -104,6 +107,7 @@ export default {
   'utils/formatNumber': formatNumber,
   'utils/mapRoutes': mapRoutes,
   'utils/withAttr': withAttr,
+  'utils/throttleDebounce': ThrottleDebounce,
   'models/Notification': Notification,
   'models/User': User,
   'models/Post': Post,