Enable GDS Editors to edit the transition date from the site dashboard

- Following a conversation with Jenny and Robin, dates can be edited
  even when the site is live.
- This specifies the format that they're entered in with the date_field
  form helper, which goes into the database as yyyy-mm-dd and gets
  converted to the GOV.UK date format upon display on the site
  dashboard, as it already did.
Remove launch_date from the redirector site YAML import

- Soon, redirector will no longer be the source of transition dates.