(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/AccountWidget.ts

@@ -8,7 +8,7 @@ import {primaryButton} from 'app/client/ui2018/buttons';
 import {colors, mediaDeviceNotSmall, testId, vars} from 'app/client/ui2018/cssVars';
 import {icon} from 'app/client/ui2018/icons';
 import {menu, menuDivider, menuItem, menuItemLink, menuSubHeader} from 'app/client/ui2018/menus';
-import {commonUrls} from 'app/common/gristUrls';
+import {commonUrls, shouldHideUiElement} from 'app/common/gristUrls';
 import {FullUser} from 'app/common/LoginSessionAPI';
 import * as roles from 'app/common/roles';
 import {Organization, SUPPORT_EMAIL} from 'app/common/UserAPI';
@@ -111,6 +111,7 @@ export class AccountWidget extends Disposable {
         // Don't show on doc pages, or for personal orgs.
         null),
 
+      shouldHideUiElement("billing") ? null :
       // Show link to billing pages.
       currentOrg && !currentOrg.owner ?
         // For links, disabling with just a class is hard; easier to just not make it a link.
@@ -128,7 +129,7 @@ export class AccountWidget extends Disposable {
 
       // In case of a single-org setup, skip all the account-switching UI. We'll also skip the
       // org-listing UI below.
-      this._appModel.topAppModel.isSingleOrg ? [] : [
+      this._appModel.topAppModel.isSingleOrg || shouldHideUiElement("multiAccounts") ? [] : [
         menuDivider(),
         menuSubHeader(dom.text((use) => use(users).length > 1 ? 'Switch Accounts' : 'Accounts')),
         dom.forEach(users, (_user) => {