(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/entity/Activation.ts

@@ -0,0 +1,17 @@
+import {BaseEntity, Column, Entity, PrimaryColumn} from "typeorm";
+
+@Entity({name: 'activations'})
+export class Activation extends BaseEntity {
+
+  @PrimaryColumn()
+  public id: string;
+
+  @Column({name: 'key', type: 'text', nullable: true})
+  public key: string|null;
+
+  @Column({name: 'created_at', default: () => "CURRENT_TIMESTAMP"})
+  public createdAt: Date;
+
+  @Column({name: 'updated_at', default: () => "CURRENT_TIMESTAMP"})
+  public updatedAt: Date;
+}