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?)
Chrome is making this date field pretty and doing client-side validation of the date, which prevents submission of an invalid date. When I edit the field type to `"text"` to simulate a less-good browser, I can submit an invalid date and it doesn't give an error. The invalid value isn't saved to the database (it becomes `NULL`) but it should instead redisplay the form with an error message.

I don't know what Postgres would do with an invalid date - it might be less happy than MySQL to silently make it NULL (is Rails doing that, or the database? too much :tada: in the :earth_africa: for me).

(Also, aaargh disappearing diffs - sorry for the multiple comments...)