[slices] add simple new slice form (#2800)

* initial structure for add new slice page

* simplify add slice form

* add a test

* fix long line

* use underscore for template name

* fix controls path

* fix vis types select

superset/assets/spec/javascripts/addSlice/AddSliceContainer_spec.jsx

@@ -0,0 +1,39 @@
+import React from 'react';
+import { expect } from 'chai';
+import { describe, it, beforeEach } from 'mocha';
+import { shallow } from 'enzyme';
+import { Button } from 'react-bootstrap';
+import Select from 'react-virtualized-select';
+import AddSliceContainer from '../../../javascripts/addSlice/AddSliceContainer';
+
+const defaultProps = {
+  datasources: [
+    { label: 'my first table', value: '1__table' },
+    { label: 'another great table', value: '2__table' },
+  ],
+};
+
+describe('AddSliceContainer', () => {
+  let wrapper;
+
+  beforeEach(() => {
+    wrapper = shallow(<AddSliceContainer {...defaultProps} />);
+  });
+
+  it('uses table as default visType', () => {
+    expect(wrapper.state().visType).to.equal('table');
+  });
+
+  it('renders 2 selects', () => {
+    expect(wrapper.find(Select)).to.have.lengthOf(2);
+  });
+
+  it('renders a button', () => {
+    expect(wrapper.find(Button)).to.have.lengthOf(1);
+  });
+
+  it('formats explore url', () => {
+    const formattedUrl = '/superset/explore/table/1?form_data=%7B%22viz_type%22%3A%22table%22%7D';
+    expect(wrapper.instance().exploreUrl()).to.equal(formattedUrl);
+  });
+});