(core) make the support account configurable, and allow listing public sites

Summary:
This makes two small tweaks based on a user's questions about sharing
sites publicly for a self-managed installation:

 * The support user `support@getgrist.com` is made configurable with
   `GRIST_SUPPORT_EMAIL`. This came up because only the support user
   can share material with the special "everyone" user. This restriction
   was added to avoid spam.
 * Regardless of public sharing settings, for our SaaS we had
   decided not to list public sites to anonymous users. That is
   somewhat a question of taste, so a `GRIST_LIST_PUBLIC_SITES` flag
   is added to override this choice.

Public sharing isn't in a well polished state, and this diff doesn't
advance that, in fact it adds a new wrinkle :-/

Test Plan: existing tests pass; manual testing

Reviewers: jarek

Reviewed By: jarek

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D3663

app/client/models/AppModel.ts

@@ -18,7 +18,7 @@ import {getDefaultThemePrefs, Theme, ThemeAppearance, ThemeColors, ThemePrefs,
         ThemePrefsChecker} from 'app/common/ThemePrefs';
 import {getThemeColors} from 'app/common/Themes';
 import {getGristConfig} from 'app/common/urlUtils';
-import {getOrgName, Organization, OrgError, SUPPORT_EMAIL, UserAPI, UserAPIImpl} from 'app/common/UserAPI';
+import {getOrgName, Organization, OrgError, UserAPI, UserAPIImpl} from 'app/common/UserAPI';
 import {getUserPrefObs, getUserPrefsObs} from 'app/client/models/UserPrefs';
 import {bundleChanges, Computed, Disposable, Observable, subscribe} from 'grainjs';
 
@@ -276,7 +276,7 @@ export class AppModelImpl extends Disposable implements AppModel {
   }
 
   public isSupport() {
-    return this.currentValidUser?.email === SUPPORT_EMAIL;
+    return Boolean(this.currentValidUser?.isSupport);
   }
 
   public isBillingManager() {