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
```
This is out of scope for this pull, but `read_attribute` feels smelly.

I think we want to store a date that doesn't care about time zone (a "raw") version somewhere, and then have another method that handles the conversion.

When we went the raw date, we'll read the attribute as usual. When we want the calculated version, we'll call the other method.