(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/server/lib/TestLogin.ts

@@ -1,3 +1,4 @@
+import {SUPPORT_EMAIL} from 'app/gen-server/lib/HomeDBManager';
 import {GristLoginSystem, GristServer} from 'app/server/lib/GristServer';
 import {Request} from 'express';
 
@@ -23,7 +24,7 @@ export async function getTestLoginSystem(): Promise<GristLoginSystem> {
           // Make sure support user has a test api key if needed.
           if (process.env.TEST_SUPPORT_API_KEY) {
             const dbManager = gristServer.getHomeDBManager();
-            const user = await dbManager.getUserByLogin('support@getgrist.com');
+            const user = await dbManager.getUserByLogin(SUPPORT_EMAIL);
             if (user) {
               user.apiKey = process.env.TEST_SUPPORT_API_KEY;
               await user.save();