Update add mapping UI to accept multiples

* Switch from text input to a text area, stretch the preceeding
bootstrap add-on to match the textarea’s height
* Update copy to refer to multiples where appropriate
* Update title to match button text, ie ‘Add’ rather than ‘New’
* Update breadcrumb and tests too
* Make edit and add page titles match the title convention by including
the hostname in the h1
Cleanup error messaging and styles

* Use bootstrap error styles rather than rails ones
* When only one error, use a paragraph rather than an unordered list
* Make sure that the Old URLs block label gets wrapped correctly when
there’s an error
* Use smart quotes in error messages
Update add mapping copy to be plural
Reverse condition for before_filter

This is shorter and clearer, especially as we add more editing actions
to the mappings controller.
Remove duplicate 'private' from mappings controller

And fix indentation according to our Ruby style guide.
Add routes and skeleton actions for bulk adding

And test that the actions have the check_user_can_edit before_filter.
Create separate view for new_multiple

This includes a new_multiple breadcrumb but no form here yet.
Add form to submit multiple paths for new mappings
Rename bulk to bulk_edit on mappings controller
Add a BulkAdder with a method to extract paths
Canonicalize paths extracted from input
Display a confirmation for multiple new mappings

This canonicalizes paths, but doesn't yet handle validation errors
sensibly or create mappings.
Display error messages at the top of the form

This shows all errors as a summary at the top, but doesn't yet
highlight fields. It also doesn't fill in the submitted values when
redisplaying the form.
Return error messages mapped to their fields
Highlight fields and labels for associated errors
Maintain values when redisplaying form with errors
Add a method to create multiple new mappings

This is remarkably optimistic.
Create mappings on confirmation

Paths are submitted by the confirmation form as an array of paths
rather than the multiline string from the initial form.