Allow different locales per tenant

Note that, currently, we take these settings from the database but we
don't provide a way to edit them through the admin interface, so the
locales must be manually introduced through a Rails console.

While we did consider using a comma-separated list, we're using spaces
in order to be consistent with the way we store the allowed content
types settings.

The `enabled_locales` nomenclature, which contrasts with
`available_locales`, is probably subconsciously based on similar
patterns like the one Nginx uses to enable sites.

Note that we aren't using `Setting.enabled_locales` in the globalize
initializer when setting the fallbacks. This means the following test
(which we could add to the shared globalizable examples) would fail:

```
it "Falls back to an enabled locale if the fallback is not enabled" do
  Setting["locales.default"] = "en"
  Setting["locales.enabled"] = "fr en"
  allow(I18n.fallbacks).to receive(:[]).and_return([:fr, :es])
  Globalize.set_fallbacks_to_all_available_locales

  I18n.with_locale(:fr) do
    expect(record.send(attribute)).to eq "In English"
  end
end
```

The reason is that the code making this test pass could be:

```
def Globalize.set_fallbacks_to_all_available_locales
  Globalize.fallbacks = I18n.available_locales.index_with do |locale|
    ((I18n.fallbacks[locale] & Setting.enabled_locales) + Setting.enabled_locales).uniq
  end
end
```

However, this would make it impossible to run `rake db:migrate` on new
applications because the initializer would try to load the `Setting`
model but the `settings` table wouldn't exist at that point.

Besides, this is a really rare case that IMHO we don't need to support.
For this scenario, an installation would have to enable a locale, create
records with contents in that locale, then disable that locale and have
that locale as a fallback for a language where content for that record
wasn't created. If that happened, it would be solved by creating content
for that record in every enabled language.

.rubocop.yml

@@ -574,6 +574,7 @@ RSpec/InstanceVariable:
   Enabled: true
   Exclude:
     - spec/controllers/concerns/has_orders_spec.rb
+    - spec/form_builders/translatable_form_builder_spec.rb
 
 RSpec/LetBeforeExamples:
   Enabled: true