(core) move home server into core

Summary: This moves enough server material into core to run a home server.  The data engine is not yet incorporated (though in manual testing it works when ported).

Test Plan: existing tests pass

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2552

app/gen-server/entity/BillingAccountManager.ts

@@ -0,0 +1,26 @@
+import {BaseEntity, Column, Entity, JoinColumn, ManyToOne, PrimaryGeneratedColumn} from 'typeorm';
+import {BillingAccount} from 'app/gen-server/entity/BillingAccount';
+import {User} from 'app/gen-server/entity/User';
+
+/**
+ * A list of users with the right to modify a giving billing account.
+ */
+@Entity({name: 'billing_account_managers'})
+export class BillingAccountManager extends BaseEntity {
+  @PrimaryGeneratedColumn()
+  public id: number;
+
+  @Column({name: 'billing_account_id'})
+  public billingAccountId: number;
+
+  @ManyToOne(type => BillingAccount, { onDelete: 'CASCADE' })
+  @JoinColumn({name: 'billing_account_id'})
+  public billingAccount: BillingAccount;
+
+  @Column({name: 'user_id'})
+  public userId: number;
+
+  @ManyToOne(type => User, { onDelete: 'CASCADE' })
+  @JoinColumn({name: 'user_id'})
+  public user: User;
+}