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