Add Globalize3 and support multilingual edition titles

* We've explicitly set I18n.default_locale to :en (english) so that we can
confidently refer to I18n.default_locale in our code.

* We've changed `Edition::Identifiable#update_document_slug` so that it only
updates the slug based on the english title. This is based on an assumption
that, at least for now, all editions will have a primary english translation.

* In making the change above, we've had to update
`Document#update_slug_if_possible` so that we don't generate a new slug if an
edition is saved in the presence of a new edition with the same title. This
was previously achieved by the `if title_changed?` condition in
`Edition::Identifiable#update_document_slug` but Globalize3 made that
problematic. We initially tried to put this conditional logic in
`Document#should_generate_new_friendly_id?` but that turned out to be too
awkward.

* We've also tried to clarify the test that ensures we don't inappropriately
generate new slugs.

* We've made use of the Edition#translations association (added by globalize3)
and its corresponding table, edition_translations, in order to implement the
Edition scopes that search for a title, and to avoid N+1 query tests failing.

* Although it's now possible to have an Edition with a title in multiple
languages, all scopes dealing with title are restricted to english (the
default locale).