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?)
Robin: When the box is empty, it displays the placeholder below. I'll work on adding more date formatting to make them consistent and possibly long, like on the site dashboard. ;-)

![screen shot 2014-09-10 at 17 21 20](https://cloud.githubusercontent.com/assets/355033/4221094/885e706a-3906-11e4-98bb-5a68008ca078.png)