(core) Add GRIST_UI_FEATURES env variable

Summary:
Tutorials are now hidden by default in grist-core and grist-ee, and can
be re-enabled via a new env variable, GRIST_UI_FEATURES, which accepts
a comma-separated list of UI features to enable.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D3885

app/client/ui/NotifyUI.ts

@@ -10,7 +10,7 @@ import {theme, vars} from 'app/client/ui2018/cssVars';
 import {icon} from 'app/client/ui2018/icons';
 import {IconName} from "app/client/ui2018/IconList";
 import {menuCssClass} from 'app/client/ui2018/menus';
-import {commonUrls, shouldHideUiElement} from 'app/common/gristUrls';
+import {commonUrls, isFeatureEnabled} from 'app/common/gristUrls';
 import {dom, makeTestId, styled} from 'grainjs';
 import {cssMenu, defaultMenuOptions, IOpenController, setPopupToCreateDom} from 'popweasel';
 
@@ -159,7 +159,7 @@ function buildNotifyDropdown(ctl: IOpenController, notifier: Notifier, appModel:
     cssDropdownContent(
       cssDropdownHeader(
         cssDropdownHeaderTitle(t("Notifications")),
-        shouldHideUiElement("helpCenter") ? null :
+        !isFeatureEnabled("helpCenter") ? null :
         cssDropdownFeedbackLink(
           cssDropdownFeedbackIcon('Feedback'),
           t("Give feedback"),