Add some tolerances to breadcrumb scrolling

See https://github.com/vector-im/riot-web/issues/9400
See https://github.com/vector-im/riot-web/issues/9394

Tolerances are defined as a device-only setting to give advanced users an option to override the values. No UI is exposed for this. 

The default values are picked for assumptions on comfort, however as people change the tolerances themselves the defaults may need to change.

src/settings/Settings.js

@@ -262,6 +262,13 @@ export const SETTINGS = {
         supportedLevels: ['account'],
         default: [],
     },
+    "breadcrumb_scroll_tolerances": {
+        supportedLevels: LEVELS_DEVICE_ONLY_SETTINGS,
+        default: {
+            xyThreshold: 10,
+            yReduction: 0.8,
+        },
+    },
     "analyticsOptIn": {
         supportedLevels: LEVELS_DEVICE_ONLY_SETTINGS_WITH_CONFIG,
         displayName: _td('Send analytics data'),