redux: When logging, show Immutable parts of state as plain JS objects.

Immutable.JS objects come with extra properties and hierarchies that
are an obstacle to debugging; we only care about the data the
objects contain. Redux themselves point out the obstacle [0], but I
noticed it in my own debugging before I saw that doc. Redux
recommends a browser extension, but I think it's better to build a
solution into our code.

So, do a simple transformation in our `redux-logger` middleware,
with an implementation informed by a `redux-logger` doc [1].

That example uses `Iterable.isIterable`, which seems like the thing
people did before `Iterable.isImmutable` arrived [2]. The
`isImmutable` doc [3] says, "True if `maybeImmutable` is an
Immutable Collection or Record".

I checked, and Collection [4] and Record [5] instances both have a
`.toJS` method. We don't use any Records yet, but we want to avoid
nasty surprises if we ever do.

[0] https://github.com/reduxjs/redux/blob/master/docs/recipes/UsingImmutableJS.md#difficult-to-debug

[1] https://github.com/LogRocket/redux-logger#transform-immutable-with-combinereducers

[2] https://stackoverflow.com/a/31919411/6792075; see also PR
    immutable-js/immutable-js#1113; issue
    immutable-js/immutable-js#566

[3] https://immutable-js.github.io/immutable-js/docs/#/isImmutable

[4] https://immutable-js.github.io/immutable-js/docs/#/Collection/toJS

[5] https://immutable-js.github.io/immutable-js/docs/#/Record/toJS

src/boot/store.js

@@ -240,6 +240,20 @@ function listMiddleware() {
       // and ours:
       //   https://github.com/zulip/zulip-mobile/blob/master/docs/howto/debugging.md#redux-logger
       createLogger({
+        stateTransformer: state => {
+          // Beware: errors thrown in here will silently drop actions.
+          const newState = {};
+          Object.keys(state).forEach(key => {
+            if (Immutable.isImmutable(state[key])) {
+              // Make things like `state.narrows` appear as plain
+              // JS objects in the logs (much easier to read)
+              newState[key] = state[key].toJS();
+            } else {
+              newState[key] = state[key];
+            }
+          });
+          return newState;
+        },
         duration: true,
         // Example options to add for more focused information, depending on
         // what you're investigating; see docs/howto/debugging.md (link above).