Enable GDS Editors to edit the transition date

- 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.
- This also removes launch_date from the redirector site YAML import
  as redirector will soon be retired as the canonical source of transition dates.

(This is related to https://github.com/alphagov/redirector/pull/918, but this one should probably be merged first?)
For those of us from America with British Date accessibility issues, can we have it read / input in long / multiple forms? [Also, does anyone know how to get the NHS to reverse a regsitered date of birth from month-to-day? ]