Make App.js into skeleton for GraphExplorer (#70)

* Make App.js into skeleton for GraphExplorer

We make a very basic skeleton for the Graph Explorer as a basis
for future development.

This commit also removes the UserExplorer and FileExplorer from
App.js. Since we have changed the underlying data model, we are
unlikely to use the UserExplorer or FileExplorer in anything like
their current state, so they are effectively deprecated. I am deferring
removing them because it is nice to have some examples of working React
code to copy from, before the Graph Explorer is ready.

Test plan: run `yarn start`, and observe that the App displays the
words "Graph Explorer" underneath the "SourceCred Explorer" title bar.

src/explorer/App.js

@@ -1,19 +1,12 @@
 // @flow
 import React, {Component} from "react";
-import data from "./data.json";
 import "./App.css";
-import {FileExplorer} from "./FileExplorer.js";
-import {UserExplorer} from "./UserExplorer.js";
+import {GraphExplorer} from "./GraphExplorer";
 
-type AppState = {selectedPath: string, selectedUser: ?string};
-class App extends Component<{}, AppState> {
-  constructor() {
-    super();
-    this.state = {
-      selectedPath: "",
-      selectedUser: null,
-    };
-  }
+type Props = {};
+type State = {};
+
+class App extends Component<Props, State> {
   render() {
     return (
       <div className="App" style={{backgroundColor: "#eeeeee"}}>
@@ -28,19 +21,8 @@ class App extends Component<{}, AppState> {
         >
           <h1 style={{fontSize: "1.5em"}}>SourceCred Explorer</h1>
         </header>
-        <FileExplorer
-          className="file-explorer"
-          onSelectPath={(x) => this.setState({selectedPath: x})}
-          selectedPath={this.state.selectedPath}
-          data={data}
-        />
-        <UserExplorer
-          className="user-explorer"
-          selectedPath={this.state.selectedPath}
-          selectedUser={this.state.selectedUser}
-          onSelectUser={(x) => this.setState({selectedUser: x})}
-          data={data}
-        />
+
+        <GraphExplorer />
       </div>
     );
   }

src/explorer/GraphExplorer.js

@@ -0,0 +1,17 @@
+// @flow
+// A frontend for visualizing Contribution Graphs
+
+import React, {Component} from "react";
+
+type Props = {};
+type State = {};
+
+export class GraphExplorer extends Component<Props, State> {
+  render() {
+    return (
+      <div>
+        <h1>Graph Explorer</h1>
+      </div>
+    );
+  }
+}