(core) clean up a collection of small problems affecting grist-core

Summary:
 * Remove adjustSession hack, interfering with loading docs under saml.
 * Allow the anonymous user to receive an empty list of workspaces for
   the merged org.
 * Behave better on first page load when org is in path - this used to
   fail because of lack of cookie.  This is very visible in grist-core,
   as a failure to load localhost:8484 on first visit.
 * Mark cookie explicitly as SameSite=Lax to remove a warning in firefox.
 * Make errorPages available in grist-core.

This changes the default behavior of grist-core to now start off in
anonymous mode, with an explicit sign-in step available.  If SAML is not configured,
the sign-in operation will unconditionally sign the user in as a default
user, without any password check or other security.  The user email is
taken from GRIST_DEFAULT_EMAIL if set.  This is a significant change, but
makes anonymous mode available in grist-core (which is convenient
for testing) and makes behavior with and without SAML much more consistent.

Test Plan: updated test; manual (time to start adding grist-core tests though!)

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2980

test/nbrowser/gristUtils.ts

@@ -1617,6 +1617,8 @@ export async function openUserProfile() {
   // Since the AccountWidget loads orgs and the user data asynchronously, the menu
   // can expand itself causing the click to land on a wrong button.
   await waitForServer();
+  await driver.findWait('.test-usermenu-org', 1000);
+  await driver.sleep(250);  // There's still some jitter (scroll-bar? other user accounts?)
   await driver.findContent('.grist-floating-menu li', 'Profile Settings').click();
   await driver.findWait('.test-login-method', 5000);
 }