Osra208 start end dates

OSRA208
The Sponsorship model can perform its own date parsing & validations. So what about the following:

```ruby
def create
  @sponsor = Sponsor.find(params[:sponsor_id])
  orphan = Orphan.find(params[:orphan_id])
  start_date = params[:sponsorship_start_date]

  sponsorship = Sponsorship.new(sponsor: @sponsor, orphan: orphan, start_date: start_date)
  if sponsorship.valid?
    sponsorship.save!
    flash[:success] = "Sponsorship was successfully created."
    redirect_to admin_sponsor_path @sponsor
  else
    flash[:warning] = sponsorship.errors.full_messages
    redirect_to new_sponsorship_path(@sponsor, scope: 'eligible_for_sponsorship')
  end
end
```
This eliminates code duplication, as well as rendering all error messages and not only those related to the date. For a clumsy but passable way to format the errors in the flash, see https://github.com/AgileVentures/osra/blob/develop/app/admin/pending_orphan_list.rb#L71, although it should not be necessary here.