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?)
This is a bit implementation-driven, can we push this detail down to the steps? I'd prefer to see something like 

```gherkin
When I edit a site's date
Then I am redirected to the site's dashboard
And the updated date should be visible
```

I do appreciate we do this lots everywhere else, and many of the implementation-driven steps are down to me, and possibly it's the case that there are too many broken windows for this project to recover :wink: