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?)
The old date is displayed in a different format from the new one, which is a bit confusing:

![screen shot 2014-09-10 at 16 34 56](https://cloud.githubusercontent.com/assets/1822424/4220238/24b67aae-3900-11e4-9565-ad5c87bd4db5.png)

I think the 21/02/2013 format would be better for both here.