cmd, dashboard: dashboard using React, Material-UI, Recharts (#15393)

* cmd, dashboard: dashboard using React, Material-UI, Recharts

* cmd, dashboard, metrics: initial proof of concept dashboard

* dashboard: delete blobs

* dashboard: gofmt -s -w .

* dashboard: minor text and code polishes

dashboard/assets/components/Common.jsx

@@ -0,0 +1,52 @@
+// Copyright 2017 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+// isNullOrUndefined returns true if the given variable is null or undefined.
+export const isNullOrUndefined = variable => variable === null || typeof variable === 'undefined';
+
+export const LIMIT = {
+    memory:  200, // Maximum number of memory data samples.
+    traffic: 200, // Maximum number of traffic data samples.
+    log:     200, // Maximum number of logs.
+};
+// The sidebar menu and the main content are rendered based on these elements.
+export const TAGS = (() => {
+    const T = {
+        home:         { title: "Home", },
+        chain:        { title: "Chain", },
+        transactions: { title: "Transactions", },
+        network:      { title: "Network", },
+        system:       { title: "System", },
+        logs:         { title: "Logs", },
+    };
+    // Using the key is circumstantial in some cases, so it is better to insert it also as a value.
+    // This way the mistyping is prevented.
+    for(let key in T) {
+        T[key]['id'] = key;
+    }
+    return T;
+})();
+
+export const DATA_KEYS = (() => {
+    const DK = {};
+    ["memory", "traffic", "logs"].map(key => {
+       DK[key] = key;
+    });
+    return DK;
+})();
+
+// Temporary - taken from Material-UI
+export const DRAWER_WIDTH = 240;