[EditCollectionPage] Left navigation pages prop not passed if URL is accessed directly

[kwajiehao]

To render the left navigation bar in a Collections page accurately, we need to know the titles of all the pages in the collection. Right now, this is done by passing a prop pages into the link in CollectionPages like so:

 <Link to={{
     pathname: `/sites/${siteName}/collections/${collectionName}/${page.fileName}`,
     state: {
         pages,
     },
}}
>

And it is subsequently accessed in EditCollectionPage:

const { match, location } = this.props;
const { pages } = location.state;

However, if EditCollectionPage is accessed by its URL and not by the link, then the pages prop is not passed and { pages } becomes undefined in EditCollectionPage.

This should be fixed by calling the Github API directly in EditCollectionPage rather than passing a prop accessible only when the link is used.

[kwajiehao]

Resolved in #64 by making a call to the collections endpoint to get all pages in the collection