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.
Remove launch_date from the redirector site YAML import

- Soon, redirector will no longer be the source of transition dates.
Add breadcrumbs and a page title to the form page

- Also add a line break for the "Edit date" text on the dashboard.
Add a check_user_is_gds_editor method to avoid multiple ifs

- Then we check GDS Editor permissions on both the edit and update
  actions.
- This is on SitesController because of its redirection to the site
  dashboard.
Format the old and new dates consistently on the edit page

- Format as 'Day Month Year' in its long form, i.e. 20 September 2014,
  in GOV.UK style.
- Use the date picker field Rails helper I found about about from
  Whitehall to make it easier for people to tell the difference between
  months and days - it displays as three separate dropdowns:
  `2014 September 11` for example.
Make these features less implementation-specific
Make the edit page look like the other pages with forms

- Add a page title and make the domain name smaller.
- Remove colons from labels.
- Add a line break between labels and their text inputs.
- Correct the breadcrumb to "Edit site".
- Use Bootstrap-styled dropdowns.
Merge pull request #396 from alphagov/make_transition_date_editable

Enable GDS Editors to edit the transition date