feat(v2): implement theme component overriding (#1435)

* feat(v2): implement component overriding

* siteDir theme overriding should work for > 1 level directory

* fallback for essential component like Loading

* rename default -> classic

packages/docusaurus-plugin-content-blog/src/theme/BlogListPage/index.js

@@ -0,0 +1,38 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import React from 'react';
+
+import Layout from '@theme/Layout'; // eslint-disable-line
+import BlogPostItem from '@theme/BlogPostItem';
+
+function BlogListPage(props) {
+  const {
+    metadata: {posts = []},
+    entries: BlogPosts,
+  } = props;
+
+  return (
+    <Layout title="Blog">
+      <div className="container margin-vert--xl">
+        <div className="row">
+          <div className="col col--6 col--offset-3">
+            {BlogPosts.map((PostContent, index) => (
+              <div className="margin-bottom--xl" key={index}>
+                <BlogPostItem truncated metadata={posts[index]}>
+                  <PostContent />
+                </BlogPostItem>
+              </div>
+            ))}
+          </div>
+        </div>
+      </div>
+    </Layout>
+  );
+}
+
+export default BlogListPage;