(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/Group.ts

@@ -0,0 +1,33 @@
+import {BaseEntity, Column, Entity, JoinTable, ManyToMany, OneToOne, PrimaryGeneratedColumn} from "typeorm";
+
+import {AclRule} from "./AclRule";
+import {User} from "./User";
+
+@Entity({name: 'groups'})
+export class Group extends BaseEntity {
+
+  @PrimaryGeneratedColumn()
+  public id: number;
+
+  @Column()
+  public name: string;
+
+  @ManyToMany(type => User)
+  @JoinTable({
+    name: 'group_users',
+    joinColumn: {name: 'group_id'},
+    inverseJoinColumn: {name: 'user_id'}
+  })
+  public memberUsers: User[];
+
+  @ManyToMany(type => Group)
+  @JoinTable({
+    name: 'group_groups',
+    joinColumn: {name: 'group_id'},
+    inverseJoinColumn: {name: 'subgroup_id'}
+  })
+  public memberGroups: Group[];
+
+  @OneToOne(type => AclRule, aclRule => aclRule.group)
+  public aclRule: AclRule;
+}