(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/Activations.ts

@@ -0,0 +1,28 @@
+import { makeId } from 'app/server/lib/idUtils';
+import { Activation } from 'app/gen-server/entity/Activation';
+import { HomeDBManager } from 'app/gen-server/lib/HomeDBManager';
+
+/**
+ * Manage activations. Not much to do currently, there is at most one
+ * activation. The activation singleton establishes an id and creation
+ * time for the installation.
+ */
+export class Activations {
+  constructor(private _db: HomeDBManager) {
+  }
+
+  // Get the current activation row, creating one if necessary.
+  // It will be created with an empty key column, which will get
+  // filled in once an activation key is presented.
+  public current(): Promise<Activation> {
+    return this._db.connection.manager.transaction(async manager => {
+      let activation = await manager.findOne(Activation);
+      if (!activation) {
+        activation = manager.create(Activation);
+        activation.id = makeId();
+        await activation.save();
+      }
+      return activation;
+    });
+  }
+}