Backfill missing entry dates

We added `date` to `Entry` in https://github.com/codecation/trailmix/pull/82 and now we get to backfill our existing entries with a date! After we've run this transition, we can add our `not null` constraint and remove our hack in [`entry.rb#L11-13`](https://github.com/codecation/trailmix/blob/64f2ef80e315960309a48465d77d47acc7888c9a/app/models/entry.rb#L11-13)

We'll run this in the console with:

```ruby
> FillInDateTransition.new.perform
```

It'll update ~15,000 entries:

```ruby
> Entry.where(date: nil).count
#=> 15233
```