Changing report download titles in include download date

Simple update to the report titles upon CSV download to include YYYYMMDD download date.

Refers to Issue 285
https://github.com/openfoodfoundation/openfoodnetwork/issues/285


Looks good! There's a couple of things we could do to neaten it up:

1. Reduce duplication by extracting the timestamp generation to a private method

```ruby
  # In the private section at the bottom of the class...
  private

  # ...

  def timestamp
    Time.now.strftime("%Y%m%d")
  end
```
2. Build the filename using a more Rubyish idiom. Most Ruby code leans toward interpolation "#{...}" syntax over string concatenation syntax.

```ruby
render_report(@report.header, @report.table, params[:csv], "customers_#{timestamp}.csv")
```