Update admin gem to 1.0.0
Replace silent margin classes with mixins

* Mixins aren’t using !important
* Generated CSS is cleaner
* Silent classes have been removed from admin gem
Explicitly pad list group items on site dashboard

* Before the default Bootstrap styles were being modified
* Instead use new admin gem helper class
Force link styles in filters to inherit colour

* Increased specificity of visited link styles in admin gem caused
filter options to go blue, prevent this from happening again
Change the remove_global_http_status_column migration to be reversible

- Following me making this only a `change` migration in #319, a discussion
  happened in which it was decided that it is important that it is
  reversible, otherwise things break in the event of an aborted deploy
  if the column removed does not rollback to exist again.
Merge pull request #317 from alphagov/upgrade-admin-gem

Upgrade admin gem to 1.0.0
Merge pull request #325 from alphagov/make_global_http_status_removal_migration_reversible

Change the remove_global_http_status_column migration to be reversible
Create BulkAddBatch and use it for bulk add

This lets us add a second subclass for the ImportBatch which will have
different inputs and behaviour.
BulkAddBatch should create BulkAddBatchEntries
Basic ImportBatch#create_entries
Add ImportBatchRow

This will represent a row in the import CSV/TSV. It will act like a presenter,
encapsulating knowledge of how to translate the row into a ImportBatchEntry.
Add a basic ImportBatchesController
Add basic specs for ImportBatchesController

Also move the shared examples for a user without permission into
AuthenticationControllerHelpers.
ImportBatchesController creates a batch
Rename @import to @batch for the #new action

This makes the instance variables consistent with the #create action.
Add an import preview page

This displays counts of each type of mapping to be created, and a
table which currently contains all of the batch's entries (rather
than the first 20).

The table duplicates small parts of the _mappings_table partial, but
since that partial includes many behaviours we don't need here it is
simpler to have this duplication instead of trying to make the partial
usable in this context as well.

This also adds a feature for the happy path (at the moment it only goes
as far as the preview page).
Handle the old URL being an absolute URL
Canonicalize the old URL

ImportBatchRow needs the site to do this.