fix(build): output aliased cms.js w/ deprecation (#1526)

When loading Netlify CMS via script tag from a CDN, the file was named
`cms.js` until 2.0, when it was renamed to `netlify-cms.js` in alignment
with all packages outputting files that match the package name. To avoid
a lot of broken sites and confusion, this commit outputs both filenames
and prints a deprecation warning to the console in `cms.js` only.

packages/netlify-cms/scripts/deprecate-old-dist.js

@@ -0,0 +1 @@
+console.warn('The `cms.js` file is deprecated and  will be removed in the next major release. Please use `netlify-cms.js` instead.');

packages/netlify-cms/webpack.config.js

@@ -1,8 +1,27 @@
 const path = require('path');
 const coreWebpackConfig = require('../netlify-cms-core/webpack.config.js');
 
-module.exports = {
-  ...coreWebpackConfig,
-  context: path.join(__dirname, 'src'),
-  entry: './index.js',
-};
+module.exports = [
+  {
+    ...coreWebpackConfig,
+    context: path.join(__dirname, 'src'),
+    entry: './index.js',
+  },
+
+  /**
+   * Output the same script a second time, but named `cms.js`, and with a
+   * deprecation notice.
+   */
+  {
+    ...coreWebpackConfig,
+    context: path.join(__dirname, 'src'),
+    entry: [
+      ...coreWebpackConfig.entry,
+      path.join(__dirname, 'scripts/deprecate-old-dist.js'),
+    ],
+    output: {
+      ...coreWebpackConfig.output,
+      filename: 'dist/cms.js',
+    },
+  },
+];