Happychat: Persist transcripts between reloads

[jordwest]

This stores the last 30 messages between refreshes, showing the recent message history before the transcript arrives from the server. It will also persist between sessions, so the user can see the most recent messages from any previous session.

How to test

1. Go to http://calyso.localhost:3000/me/chat
2. Send > 30 messages
3. Open the Redux devtools
4. Refresh the page
5. At first, the 30 most recent messages should be shown. Soon after, you should see the older messages appear in the timeline once they're fetched from the server. Ensure messages are in the correct order.
6. Check the Redux state history to confirm. At bootup, there should have been 30 messages in the state. Once the HAPPYCHAT_TRANSCRIPT_RECEIVE action fired, the happychat.timeline should show the full transcript of the recent session.

[matticbot]

Test live: https://calypso.live/?branch=update/happychat/fetch-transcripts

[matticbot]

@jordwest This PR needs a rebase

[mattwondra]

@jordwest Is this PR still relevant? It branches off update/happychat/refactor-with-middleware-2 which was closed in #11604

[jordwest]

This one is still relevant, but will need a few changes/rebasing off master now that some middleware is in place.

[jordwest]

This is now ready for testing and review - the scope is reduced and it's much simpler now that the transcript fetching is already working in master.

[mattwondra]

👍 Looks and works great! I'll plan to 🚢 this for you early next week.

[jordwest]

Thanks for looking after this one @mattwondra!