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/components/shared/globalize_locales_component_spec.rb

@@ -0,0 +1,49 @@
+require "rails_helper"
+
+describe Shared::GlobalizeLocalesComponent do
+  describe "Language selector" do
+    it "only includes enabled locales" do
+      Setting["locales.enabled"] = "en nl"
+
+      I18n.with_locale(:en) do
+        render_inline Shared::GlobalizeLocalesComponent.new
+
+        expect(page).to have_select options: ["Choose language", "English"]
+      end
+
+      I18n.with_locale(:es) do
+        render_inline Shared::GlobalizeLocalesComponent.new
+
+        expect(page).to have_select options: ["Seleccionar idioma"]
+      end
+    end
+  end
+
+  describe "links to destroy languages" do
+    it "only includes enabled locales" do
+      Setting["locales.enabled"] = "en nl"
+
+      I18n.with_locale(:en) do
+        render_inline Shared::GlobalizeLocalesComponent.new
+
+        expect(page).to have_css "a[data-locale]", count: 1
+      end
+
+      I18n.with_locale(:es) do
+        render_inline Shared::GlobalizeLocalesComponent.new
+
+        expect(page).not_to have_css "a[data-locale]"
+      end
+    end
+  end
+
+  describe "Add language selector" do
+    it "only includes enabled locales" do
+      Setting["locales.enabled"] = "en nl"
+
+      render_inline Shared::GlobalizeLocalesComponent.new
+
+      expect(page).to have_select options: ["Add language", "English", "Nederlands"]
+    end
+  end
+end