Cleanup of Ownership directory (#2120)

* Remove Ownable.isOwner.

* Remove ownable behavior from tests

* Make Escrow use Ownable instead of Secondary

* Migrate GSNRecipientERC20Fee to Ownable

* Remove Secondary

* Move Ownable to access directory

* Remove mentions of Secondary

* Add changelog entry

* Move Ownable tests to access

* Remove unused mock

test/access/Ownable.test.js

@@ -0,0 +1,54 @@
+const { accounts, contract } = require('@openzeppelin/test-environment');
+const { constants, expectEvent, expectRevert } = require('@openzeppelin/test-helpers');
+const { ZERO_ADDRESS } = constants;
+
+const { expect } = require('chai');
+
+const Ownable = contract.fromArtifact('OwnableMock');
+
+describe('Ownable', function () {
+  const [ owner, other ] = accounts;
+
+  beforeEach(async function () {
+    this.ownable = await Ownable.new({ from: owner });
+  });
+
+  it('should have an owner', async function () {
+    expect(await this.ownable.owner()).to.equal(owner);
+  });
+
+  it('changes owner after transfer', async function () {
+    const receipt = await this.ownable.transferOwnership(other, { from: owner });
+    expectEvent(receipt, 'OwnershipTransferred');
+
+    expect(await this.ownable.owner()).to.equal(other);
+  });
+
+  it('should prevent non-owners from transferring', async function () {
+    await expectRevert(
+      this.ownable.transferOwnership(other, { from: other }),
+      'Ownable: caller is not the owner'
+    );
+  });
+
+  it('should guard ownership against stuck state', async function () {
+    await expectRevert(
+      this.ownable.transferOwnership(ZERO_ADDRESS, { from: owner }),
+      'Ownable: new owner is the zero address'
+    );
+  });
+
+  it('loses owner after renouncement', async function () {
+    const receipt = await this.ownable.renounceOwnership({ from: owner });
+    expectEvent(receipt, 'OwnershipTransferred');
+
+    expect(await this.ownable.owner()).to.equal(ZERO_ADDRESS);
+  });
+
+  it('should prevent non-owners from renouncement', async function () {
+    await expectRevert(
+      this.ownable.renounceOwnership({ from: other }),
+      'Ownable: caller is not the owner'
+    );
+  });
+});