(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/common/ShareAnnotator.ts

@@ -55,11 +55,11 @@ export class ShareAnnotator {
     }
     const top = features.maxSharesPerDoc;
     let at = 0;
-    const makeAnnotation = (user: {email: string, isMember?: boolean, access: string|null}) => {
+    const makeAnnotation = (user: {email: string, isMember?: boolean, isSupport?: boolean, access: string|null}) => {
       const annotation: ShareAnnotation = {
         isMember: user.isMember,
       };
-      if (user.email === 'support@getgrist.com') {
+      if (user.isSupport) {
         return { isSupport: true };
       }
       if (!annotation.isMember && user.access) {