Adds an order field to the poem to enable quick random selection (and removes the indices for the entry_keys to make up for the added write op).
Renames Entry.id to Entry.order, changing it to a continuously re-computed property so that the order of entries is constantly changing, preventing 'starvation'. Because it changes constantly, I also needed to change how encoded_ids works (which used the old order field for lookups), so instead I encoded the numeric id generated by AppEngine for the model's Key, which remains constant over time.