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/admin/site_customization/content_blocks/form_content_block_component_spec.rb

@@ -0,0 +1,18 @@
+require "rails_helper"
+
+describe Admin::SiteCustomization::ContentBlocks::FormContentBlockComponent do
+  describe "locale selector" do
+    let(:content_block) { create(:site_customization_content_block, locale: "de") }
+    let(:component) do
+      Admin::SiteCustomization::ContentBlocks::FormContentBlockComponent.new(content_block)
+    end
+
+    it "only includes enabled settings" do
+      Setting["locales.enabled"] = "de fr"
+
+      render_inline component
+
+      expect(page).to have_select "locale", options: ["de", "fr"]
+    end
+  end
+end

spec/components/admin/site_customization/content_blocks/form_heading_content_block_component_spec.rb

@@ -0,0 +1,18 @@
+require "rails_helper"
+
+describe Admin::SiteCustomization::ContentBlocks::FormHeadingContentBlockComponent do
+  describe "locale selector" do
+    let(:content_block) { create(:heading_content_block, locale: "de") }
+    let(:component) do
+      Admin::SiteCustomization::ContentBlocks::FormHeadingContentBlockComponent.new(content_block)
+    end
+
+    it "only includes enabled settings" do
+      Setting["locales.enabled"] = "de fr"
+
+      render_inline component
+
+      expect(page).to have_select "locale", options: ["de", "fr"]
+    end
+  end
+end

spec/components/admin/site_customization/information_texts/form_component_spec.rb

@@ -0,0 +1,29 @@
+require "rails_helper"
+
+describe Admin::SiteCustomization::InformationTexts::FormComponent do
+  describe "enabled_translations fields" do
+    it "renders fields for enabled locales" do
+      Setting["locales.enabled"] = "en es"
+      content = create(:i18n_content)
+
+      render_inline Admin::SiteCustomization::InformationTexts::FormComponent.new([[content]])
+
+      expect(page).to have_css "input[name^='enabled_translations']", count: 2, visible: :all
+      expect(page).to have_css "input[name='enabled_translations[en]']", visible: :hidden
+      expect(page).to have_css "input[name='enabled_translations[es]']", visible: :hidden
+    end
+  end
+
+  describe "text fields" do
+    it "renders fields for enabled locales" do
+      Setting["locales.enabled"] = "en es"
+      content = create(:i18n_content, key: "system.failure")
+
+      render_inline Admin::SiteCustomization::InformationTexts::FormComponent.new([[content]])
+
+      expect(page).to have_css "textarea[name^='contents[content_system.failure]']", count: 2, visible: :all
+      expect(page).to have_field "contents[content_system.failure]values[value_en]"
+      expect(page).to have_field "contents[content_system.failure]values[value_es]"
+    end
+  end
+end