Move left pane entirely to React

ts/util/makeLookup.ts

@@ -0,0 +1,12 @@
+import { fromPairs, map } from 'lodash';
+
+export function makeLookup<T>(
+  items: Array<T>,
+  key: string
+): { [key: string]: T } {
+  // Yep, we can't index into item without knowing what it is. True. But we want to.
+  // @ts-ignore
+  const pairs = map(items, item => [item[key], item]);
+
+  return fromPairs(pairs);
+}