Paginated feeds should specify a "nextToken" to guide the UI as to how to get the next page

The value of the `nextToken` should be what would be expected as the "start" parameter to get more results, though it's probably not worth strictly validating its format.

If there are no more items left in the feed, the `nextToken` returned should be falsey.

The consolidated oae-library module in the Discussions PR shows an example of this.

Feeds todo:
 - [x] Content comments
 - [x] Content members
 - [x] Discussion members
 - [x] Discussion comments
 - [ ] Group members
 - [ ] User memberships