Disable rather than hide form fields that can’t be modified

Not clear to the user why the form field is sometimes present and
sometimes not present.

The Category tag field may only be modified if authorities have not yet
been assigned to the Category. This commit explains this and disables
the field if this is true.
Provide expectations for controller specs

This commit adds expectations to AdminPublicBodyCategoriesController and
AdminPublicBodyHeadingsController edit actions
Redirect to #edit after updating

On a successful update AdminPublicBodyHeadingsController and
AdminPublicBodyCategoriesController #update should redirect to the #edit
action rather than simply rendering the ‘edit’ template.

Fixes https://github.com/mysociety/alaveteli/issues/1977
Redirect to #edit after updating

On an unsuccessful update AdminPublicBodyHeadingsController and
AdminPublicBodyCategoriesController #update should render the #edit
action.
Add expectation to #index spec

The spec that covers AdminPublicBodyCategoriesController#index did not
have an expectation
Add coverage to ensure ‘new’ templates are rendered
Add specs to cover unsuccessful creation
Adds button missing from 921ee9a19c267bb0e6e784091b985f3ba80d835b
Use flash[:error] for error condition

Also use double quotes rather than string escaping
Merge branch '1977-update-categories-redirect-to-edit' into rails-3-develop
Merge branch '1968-disable-category-tag-field' into rails-3-develop
Use badge to show number of categories in heading

The chevron didn't illustrate that the heading had any category
children. The app uses the badge pattern elsewhere.

Adds minor style improvements:

  - Pad erb tags
  - Use each instead of for

Fixes https://github.com/mysociety/alaveteli/issues/1962
Remove stubs of removed method

PublicBodyCategory#load_categories no longer exists, so don't try to
stub it.

Fixes https://github.com/mysociety/alaveteli/issues/1970
Add CensorRule factories
Specs for AdminCensorRuleController#new
Specs for AdminCensorRuleController#edit
Specs for AdminCensorRuleController#destroy

Note that these specs describe what the system does – not what it
_should_ do!
Specs for AdminCensorRuleController#create

Note that these specs describe what the system does – not what it
_should_ do!
Specs for AdminCensorRuleController#update

Note that these specs describe what the system does – not what it
_should_ do!
Initialize a new CensorRule in #new