Add custom logger. Fix a bunch of errors. Optimize imports.

backend/src/logger.ts

@@ -0,0 +1,23 @@
+// tslint-disable:no-console
+
+export const logger = {
+  info(...args: Parameters<typeof console.log>) {
+    console.log("[INFO]", ...args);
+  },
+
+  warn(...args: Parameters<typeof console.warn>) {
+    console.warn("[WARN]", ...args);
+  },
+
+  error(...args: Parameters<typeof console.error>) {
+    console.error("[ERROR]", ...args);
+  },
+
+  debug(...args: Parameters<typeof console.log>) {
+    console.log("[DEBUG]", ...args);
+  },
+
+  log(...args: Parameters<typeof console.log>) {
+    console.log(...args);
+  },
+};