multi: Add Proposal Loader + Store Listeners

[victorgcramos]

This PR adds loading placeholders for loading proposals. Fixes #2748 (comment)

It also introduces the new useStoreEffect hook, which uses the also implemented addStoreEffect listener to our Store.

Usage

You can use the addStoreEffect outside the React scope, and trigger a side effect when some action is dispatched. In the following example, we will add a listener, using the regarded method, to log a "Hello" message on the console after api is fetched:

addStoreEffect(() => {
  console.log("Hello!");
}, [api.fetch.fulfilled]);

Easy, huh?

If you're using it on some react component, and don't want it to be triggered on every render, you can use the useStoreEffect hook. In the following example, we will fetch records and ticketvote policies after api is fetched:

 useStoreEffect(() => {
    dispatch(ticketvotePolicy.fetch());
    dispatch(recordsPolicy.fetch());
  }, [api.fetch.fulfilled]);

[tiagoalvesdulce]

LGTM