Re-add dashboard short links (#5398)

* Re-add dashboard short links

* Make the short link available to all users
* Include filters in the short link for dashboards

* Remove duplicate key causing linter error

* Change URL Short link button into a menu item with Modal

* Split out tests, bind URL short link in constructor

superset/assets/spec/javascripts/components/URLShortLinkModal_spec.jsx

@@ -0,0 +1,27 @@
+import React from 'react';
+import configureStore from 'redux-mock-store';
+import { expect } from 'chai';
+import { describe, it } from 'mocha';
+import { shallow } from 'enzyme';
+
+import URLShortLinkModal from '../../../src/components/URLShortLinkModal';
+import ModalTrigger from '../../../src/components/ModalTrigger';
+
+describe('URLShortLinkModal', () => {
+  const defaultProps = {
+    url: 'mockURL',
+    emailSubject: 'Mock Subject',
+    emailContent: 'mock content',
+  };
+
+  function setup() {
+    const mockStore = configureStore([]);
+    const store = mockStore({});
+    return shallow(<URLShortLinkModal {...defaultProps} />, { context: { store } }).dive();
+  }
+
+  it('renders ModalTrigger', () => {
+    const wrapper = setup();
+    expect(wrapper.find(ModalTrigger)).have.length(1);
+  });
+});