Add publication format to edition view

Make it clear what type of format is being viewed. Also shorten text
used for editions and statuses.
Update to content models 27.0.0
Merge pull request #332 from alphagov/update_govuk_content_models

Update to content models 27.0.0
Merge pull request #330 from alphagov/format-on-edition

Add publication format to edition view
Bump admin gem to 1.4.2

Pull in fix for wrapping environment label
Merge pull request #331 from alphagov/fix-wrapping-env-label

Fix wrapping environment label with admin gem bump
Update content models to 27.2.0
Disable legacy 'nil' user account.

This account has no uid, name or email address and appears to be used
for unassigning content. It's no longer needed but it will have
  associations in other models so disable it.
Allow Editions to be unassigned.

Editions could formerly be assigned to a dummy user account as a
workaround to unassigning them.

- Update select2 assignee form control to allow easy unassignment.
- Update controller and workflow to allow real unassignment.
Merge pull request #333 from alphagov/unassign_user

Unassign an item in Publisher
Remove the "Archive by" column

For every archived edition the column is empty.
Merge pull request #334 from alphagov/remove-archive-by-column

Remove the "Archive by" column from archived publications table