(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/HomeLeftPane.ts

@@ -12,6 +12,7 @@ import {colors, testId} 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 {shouldHideUiElement} 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';
@@ -96,6 +97,7 @@ export function createHomeLeftPane(leftPanelOpen: Observable<boolean>, home: Hom
       )),
       cssTools(
         cssPageEntry(
+          dom.hide(shouldHideUiElement("templates")),
           cssPageEntry.cls('-selected', (use) => use(home.currentPage) === "templates"),
           cssPageLink(cssPageIcon('FieldTable'), cssLinkText("Examples & Templates"),
             urlState().setLinkUrl({homePage: "templates"}),