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.

spec/models/setting_spec.rb

@@ -258,4 +258,50 @@ describe Setting do
       expect(Setting.force_presence_postal_code?).to be true
     end
   end
+
+  describe ".available_locales" do
+    before { allow(I18n).to receive(:available_locales).and_return(%i[de en es pt-BR]) }
+
+    it "uses I18n available locales by default" do
+      Setting["locales.enabled"] = ""
+
+      expect(Setting.enabled_locales).to eq %i[de en es pt-BR]
+    end
+
+    it "defines available locales with a space-separated list" do
+      Setting["locales.enabled"] = "de es"
+
+      expect(Setting.enabled_locales).to eq %i[de es]
+    end
+
+    it "handles locales which include a dash" do
+      Setting["locales.enabled"] = "de en pt-BR"
+
+      expect(Setting.enabled_locales).to eq %i[de en pt-BR]
+    end
+
+    it "ignores extra whitespace between locales" do
+      Setting["locales.enabled"] = " de  en   pt-BR "
+
+      expect(Setting.enabled_locales).to eq %i[de en pt-BR]
+    end
+
+    it "ignores locales which aren't available" do
+      Setting["locales.enabled"] = "de es en-US fr zh-CN"
+
+      expect(Setting.enabled_locales).to eq %i[de es]
+    end
+
+    it "ignores words that don't make sense in this context" do
+      Setting["locales.enabled"] = "yes es 1234 en SuperCool"
+
+      expect(Setting.enabled_locales).to eq %i[es en]
+    end
+
+    it "uses I18n available locales when no locale is available" do
+      Setting["locales.enabled"] = "nl fr zh-CN"
+
+      expect(Setting.enabled_locales).to eq %i[de en es pt-BR]
+    end
+  end
 end