(core) revive saml support and test against Auth0

Summary:
SAML support had broken due to SameSite changes in browsers. This
makes it work again, and tests it against Auth0 (now owned by Okta).

Logging in and out works.  The logged out state is confusing, and may
not be complete.  The "Add Account" menu item doesn't work.
But with this, an important part of self-hosting becomes easier.

SAML support works also in grist-core, for site pages, but there
is a glitch on document pages that I'll look into separately.

Test Plan: tested manually

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2976

app/server/lib/Permit.ts

@@ -25,10 +25,13 @@
  *   - Optionally, remove the permit with removePermit().
  */
 export interface Permit {
-  docId?: string;
-  workspaceId?: number;
-  org?: string|number;
+  docId?: string;       // A particular document.
+  workspaceId?: number; // A particular workspace.
+  org?: string|number;  // A particular org.
   otherDocId?: string;  // For operations involving two documents.
+  sessionId?: string;   // A particular session.
+  url?: string;         // A particular url.
+  action?: string;      // A string denoting what kind of action the permit applies to.
 }
 
 /* A store of permits */
@@ -36,7 +39,7 @@ export interface IPermitStore {
 
   // Store a permit, and return the key it is stored in.
   // Permits are transient, and will expire.
-  setPermit(permit: Permit): Promise<string>;
+  setPermit(permit: Permit, ttlMs?: number): Promise<string>;
 
   // Get any permit associated with the given key, or null if none.
   getPermit(permitKey: string): Promise<Permit|null>;
@@ -48,12 +51,16 @@ export interface IPermitStore {
   close(): Promise<void>;
 }
 
+export interface IPermitStores {
+  getPermitStore(prefix: string, defaultTtlMs?: number): IPermitStore;
+}
+
 // Create a well formatted permit key from a seed string.
-export function formatPermitKey(seed: string) {
-  return `permit-${seed}`;
+export function formatPermitKey(seed: string, prefix: string) {
+  return `permit-${prefix}-${seed}`;
 }
 
 // Check that permit key is well formatted.
-export function checkPermitKey(key: string): boolean {
-  return key.startsWith('permit-');
+export function checkPermitKey(key: string, prefix: string): boolean {
+  return key.startsWith(`permit-${prefix}-`);
 }