Clean up scope of queried repos on index route

Links to additional points raised in issue #133.

From that issue:

- On the index route's query to ember-learn repos, there is no language that identifies this scope. Do we want to add language that clarifies or broaden the scope of the query to include other orgs?

- If we maintain the current card-based design of the index page, but expanded out to other orgs beyond ember-learn, this page would grow rather long unless we arbitrarily impose a limit on the number of repos returned by the query.

- per @mansona's comment: not all repos are useful right now (e.g. the cards show "forks" for each repo... but that's not really the intent of this app).

- Would much prefer to see the number of help-wanted issues (or rather the number of issues that would show up in our filters).