(core) Allow configuring (mostly hiding) various little bits of UI

Summary:
Adds two new env vars GRIST_HIDE_UI_ELEMENTS and GRIST_PAGE_TITLE_SUFFIX which translate to values in GristLoadConfig that the server sends the client when loading.

For checkin task https://gristlabs.getgrist.com/doc/check-ins/p/5#a1.s9.r1882.c19

Test Plan: Tested manually

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3449

app/client/ui/NotifyUI.ts

@@ -8,7 +8,7 @@ import {colors, 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} from 'app/common/gristUrls';
+import {commonUrls, shouldHideUiElement} from 'app/common/gristUrls';
 import {dom, makeTestId, styled} from 'grainjs';
 import {cssMenu, defaultMenuOptions, IOpenController, setPopupToCreateDom} from 'popweasel';
 
@@ -142,6 +142,7 @@ function buildNotifyDropdown(ctl: IOpenController, notifier: Notifier, appModel:
     cssDropdownContent(
       cssDropdownHeader(
         cssDropdownHeaderTitle('Notifications'),
+        shouldHideUiElement("helpCenter") ? null :
         cssDropdownFeedbackLink(
           cssDropdownFeedbackIcon('Feedback'),
           'Give feedback',