Sum up any labels called `/\d+ points?/`.

To aid ops in their Scrumgile-ish planning, this will sum up the points-values of specific labels on issues.

The tricky part was figuring out how to make it update based on filters, but that was mainly caused by my total lack of knowledge of Ember.js. The major addition here is just the `filteredIssues` function, which I copy-pasta'd from a different part of the app. I'm guessing it really belongs in a model, but I'm not sure.

While here, also make it so the existing issues-count updates based on the filtered issues-list.

r: @jduff @pallan
cc: @Shopify/ops 

This merges into a different branch I have, just to keep the diff clean. It should apply to master as well with only a little work (`application.js` mainly).