Activations: add an enabled_at column

For #1140, I considered trying to use the existing fields in a better
way, but because we already use the activations table to store
preferences, we need to keep all of the existing data and its usage
as-is.

The enterprise code will use this new column to decide how long the
trial period should be.

app/gen-server/entity/Activation.ts

@@ -22,6 +22,15 @@ export class Activation extends BaseEntity {
   @Column({name: 'updated_at', default: () => "CURRENT_TIMESTAMP"})
   public updatedAt: Date;
 
+  // When the enterprise activation was first enabled, so we know when
+  // to start counting the trial date.
+  //
+  // Activations are created at Grist installation to track other
+  // things such as prefs, but the user might not enable Enterprise
+  // until later.
+  @Column({name: 'enabled_at', type: nativeValues.dateTimeType, nullable: true})
+  public enabledAt: Date|null;
+
   public checkProperties(props: any): props is Partial<InstallProperties> {
     for (const key of Object.keys(props)) {
       if (!installPropertyKeys.includes(key)) {