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?)
OK. Whitehall uses `datetime_select`, another Rails helper, to do things like:

![screen shot 2014-09-11 at 11 04 04](https://cloud.githubusercontent.com/assets/355033/4232296/fe152ef4-399a-11e4-88d1-09f6e2c54c93.png)

I'll look into how we do this in Transition in this case, although we don't need time so we can just use `date_select`.