(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/HomeLeftPane.ts

@@ -16,7 +16,7 @@ import {testId, theme} from 'app/client/ui2018/cssVars';
 import {icon} from 'app/client/ui2018/icons';
 import {menu, menuIcon, menuItem, upgradableMenuItem, upgradeText} from 'app/client/ui2018/menus';
 import {confirmModal} from 'app/client/ui2018/modals';
-import {commonUrls, shouldHideUiElement} from 'app/common/gristUrls';
+import {commonUrls, isFeatureEnabled} from 'app/common/gristUrls';
 import * as roles from 'app/common/roles';
 import {Workspace} from 'app/common/UserAPI';
 import {computed, dom, domComputed, DomElementArg, observable, Observable, styled} from 'grainjs';
@@ -109,7 +109,7 @@ export function createHomeLeftPane(leftPanelOpen: Observable<boolean>, home: Hom
       )),
       cssTools(
         cssPageEntry(
-          dom.hide(shouldHideUiElement("templates")),
+          dom.show(isFeatureEnabled("templates")),
           cssPageEntry.cls('-selected', (use) => use(home.currentPage) === "templates"),
           cssPageLink(cssPageIcon('Board'), cssLinkText(t("Examples & Templates")),
             urlState().setLinkUrl({homePage: "templates"}),
@@ -125,7 +125,7 @@ export function createHomeLeftPane(leftPanelOpen: Observable<boolean>, home: Hom
         ),
         cssSpacer(),
         cssPageEntry(
-          dom.hide(shouldHideUiElement("templates")),
+          dom.show(isFeatureEnabled('tutorials')),
           cssPageLink(cssPageIcon('Bookmark'), cssLinkText(t("Tutorial")),
             { href: commonUrls.basicTutorial, target: '_blank' },
             testId('dm-basic-tutorial'),