adds context to new list item button

[DrianHillman]

Fixes Issue #300

- Summary
This PR adds the list label to the new button which provides users better context (especially with nested lists).

- Test plan
All tests continue to pass. I created nested lists 20 levels deep to test & ensure scalability. I ran npm build test & npm run line in compliance with contributing.md guidelines.

Example screenshot of the result here:

- Description for the changelog

• Adds the specific content type to the list control widget's "new" button.

- A picture of a cute animal (not mandatory but encouraged)

[erquhart]

@DrianHillman thanks for the fast PR! I added one change request, should be good to merge after that.

[DrianHillman]

Sure thing, @erquhart! I'd be happy to take a look at a revision, however, I don't think it posted. Can you clarify what it is?

[erquhart]

Two pointers:

• The field prop, along with most state collections in the CMS, are using Immutable data structures. In this case it's a Map, so you'll want to use get to access it's properties. Also note that you can more easily inspect/debug Immutable collections by using the toJS method, which converts the collection to a plain object or array. Again, this is just for inspection purposes.
• You can shorten things a tad by adding field to the object deconstruction in the previous line.

Applying these points, you get:

const { value, forId, field } = this.props;
const listLabel = field.get('label');

[erquhart]

Sorry about that, forgot to post. It's up now.

[Benaiah]

LGTM once @erquhart's change request is addressed.

[DrianHillman]

• Edits confirmed!

Learned a new technique today— thanks @erquhart! I appreciate the breakdown & links; this makes sense. The Immutable package looks great, it makes the access to props & data more performant and much more concise. 👍🏽

[erquhart]

@DrianHillman awesome work, man. If you're available/interested, join us on Wednesday for our first bi-weekly community planning session via Hangouts: https://www.eventbrite.com/e/netlify-cms-planning-session-bi-weekly-tickets-35794059997

[DrianHillman]

@erquhart Thanks! Wish I could make it tomorrow, but I'm all tied up at that time. It is a great idea to meet regularly about the project and stay on the same page with contributors— that's very cool, I'll keep an eye out for future ones.