Use single quotes instead of double quotes

- This makes the airbrake gem line consistent with all of the others - I
  found this when looking through to upgrade Rails to v4.x and there was
  no reason for it to be quoted differently.
Merge pull request #321 from alphagov/use_single_quotes_in_gemfile

Use single quotes instead of double quotes
Remove the global_http_status column

- This is the third and final part of using type instead of http_status
  globally.
- I got a collation diff - I've `git add -p` only the parts of my
  db/structure.sql file relevant to adding this new migration.
Make breadcrumbs and titles consistent
Follow our style for string quoting
Merge pull request #322 from alphagov/tidy_up_titles

Tidy up titles/breadcrumbs
Remove redundant relationship

The next line defines a short name for the same relationship. We only use the
short version because nobody likes extra typing.
Merge pull request #323 from alphagov/remove_redundant_relationship

Remove redundant relationship
Merge pull request #319 from alphagov/remove_global_http_status_column

Remove the global_http_status column
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).