(core) Support "Public access" switch in the UI for docs.

Summary:
- The older "Anonymous access" option is renamed to "Public access" in the UI.
- This option was only present with GRIST_SUPPORT_ANON set.
- With GRIST_SUPPORT_ANON, the old behavior is kept: "Public access"
  option adds/removes anon@ user
- Without GRIST_SUPPORT_ANON (normal case), orgs/workspaces don't support
  "Public access" option. For documents, it adds/removes everyone@ user.

The latter is the main feature of interest. The GRIST_SUPPORT_ANON flag
is set for on-premise installs, and adds discoverability by anon users.

Test Plan: Added a test cases and checks in other tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2612

app/common/UserAPI.ts

@@ -15,6 +15,9 @@ import {addCurrentOrgToPath} from 'app/common/urlUtils';
 // Nominal email address of the anonymous user.
 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';