Implemented new Permission system on the DB side.

ref #22

src/models/entities/Principal.ts

@@ -0,0 +1,24 @@
+import { IsInt, IsOptional } from 'class-validator';
+import { Entity, OneToMany, PrimaryGeneratedColumn, TableInheritance } from 'typeorm';
+import { Permission } from './Permission';
+
+/**
+ * Defines a admin user.
+*/
+@Entity()
+@TableInheritance({ column: { name: "type", type: "varchar" } })
+export abstract class Principal {
+  /**
+  * autogenerated unique id (primary key).
+  */
+  @PrimaryGeneratedColumn()
+  @IsInt()
+  id: number;
+
+  /**
+* permissions
+*/
+  @IsOptional()
+  @OneToMany(() => Permission, permission => permission.principal, { nullable: true })
+  permissions?: Permission[];
+}