(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/UserAPI.ts

@@ -23,9 +23,6 @@ export const ANONYMOUS_USER_EMAIL = 'anon@getgrist.com';
 // Nominal email address of a user who, if you share with them, everyone gets access.
 export const EVERYONE_EMAIL = 'everyone@getgrist.com';
 
-// A special user allowed to add/remove the EVERYONE_EMAIL to/from a resource.
-export const SUPPORT_EMAIL = 'support@getgrist.com';
-
 // A special 'docId' that means to create a new document.
 export const NEW_DOCUMENT_CODE = 'new';