Create collections #85

Right now the tips are in reverse creation time order, like a blog. You can also look at categories, but there are only a few broad categories, and the tips there are also in reverse creation time order.

In addition to this "feed" of tips, I'm proposing to add collection pages.

A collection page contains a collection of tips that are all relevant to a specific debugging scenario. Collections can be time-based (e.g. advent calendar of CSS debugging, perf debugging week).