user_settings: Add option to disable escape key navigation to default view.

Add `escape_navigates_to_default_view` as a bool setting in
UserBaseSettings model and implement it as a checkbox that toggles
the hotkey implementation of escape to the default view in the
advanced user display settings.

With /help/ documentation edits from Alya Abbott.

Fixes #20043.

static/js/realm_user_settings_defaults.ts

@@ -21,6 +21,7 @@ export type RealmDefaultSettingsType = {
     enable_stream_email_notifications: boolean;
     enable_stream_push_notifications: boolean;
     enter_sends: boolean;
+    escape_navigates_to_default_view: boolean;
     fluid_layout_width: boolean;
     high_contrast_mode: boolean;
     left_side_userlist: boolean;