Support pagination (again) with first letter filter

At the moment, Harvest returns all results for a set of filters at once, which can be quite a heavy operation. We need to adjust this so we have a guarantee of how many results are being shown at any given time. This should improve performance and allow us to do some other interesting things.

A list of first letters corresponding to the results would do nicely as a starting point. That is mocked up neatly here:
https://wiki.ubuntu.com/Harvest/NewUI#Current mockup

It would be nice if the first letter filter would hide itself, group together letters or add extra letters depending on the situation. The goal there should be that every clickable option should only reveal a specific number of results (perhaps a maximum of 30 at once).