(core) add minimal support for activation keys

Summary: For grist-ee, expect an activation key in environment variable `GRIST_ACTIVATION` or in a file pointed to by `GRIST_ACTIVATION_FILE`. In absence of key, start a 30-day trial, during which a banner is shown. Once trial expires, installation goes into document-read-only mode.

Test Plan: added a test

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D3426

app/gen-server/lib/HomeDBManager.ts

@@ -253,6 +253,8 @@ export class HomeDBManager extends EventEmitter {
                                    // deployments on same subdomain.
 
   private _docAuthCache = new MapWithTTL<string, Promise<DocAuthResult>>(DOC_AUTH_CACHE_TTL);
+  // In restricted mode, documents should be read-only.
+  private _restrictedMode: boolean = false;
 
   public emit(event: NotifierEvent, ...args: any[]): boolean {
     return super.emit(event, ...args);
@@ -338,6 +340,10 @@ export class HomeDBManager extends EventEmitter {
     this._idPrefix = prefix;
   }
 
+  public setRestrictedMode(restricted: boolean) {
+    this._restrictedMode = restricted;
+  }
+
   public async connect(): Promise<void> {
     try {
       // If multiple servers are started within the same process, we
@@ -1104,7 +1110,7 @@ export class HomeDBManager extends EventEmitter {
       if (docs.length > 1) { throw new ApiError('ambiguous document request', 400); }
       doc = docs[0];
       const features = doc.workspace.org.billingAccount.product.features;
-      if (features.readOnlyDocs) {
+      if (features.readOnlyDocs || this._restrictedMode) {
         // Don't allow any access to docs that is stronger than "viewers".
         doc.access = roles.getWeakestRole('viewers', doc.access);
       }