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.

app/components/admin/site_customization/content_blocks/form_content_block_component.html.erb

@@ -6,7 +6,7 @@
     <%= f.select :name, options_for_select(valid_blocks, selected_content_block) %>
   </div>
   <div class="small-12 medium-6 column">
-    <%= f.select :locale, I18n.available_locales %>
+    <%= f.select :locale, Setting.enabled_locales %>
   </div>
 
   <div class="small-12 column">

app/components/admin/site_customization/content_blocks/form_heading_content_block_component.html.erb

@@ -7,7 +7,7 @@
   </div>
   <div class="small-12 medium-6 column">
     <%= label_tag :locale %>
-    <%= select_tag :locale, options_for_select(I18n.available_locales, content_block.locale.to_sym) %>
+    <%= select_tag :locale, options_for_select(Setting.enabled_locales, content_block.locale.to_sym) %>
   </div>
   <div class="small-12 column">
     <%= label_tag :body %>

app/components/admin/site_customization/information_texts/form_component.html.erb

@@ -1,13 +1,13 @@
 <%= render Shared::GlobalizeLocalesComponent.new %>
 
 <%= form_tag admin_site_customization_information_texts_path do %>
-  <% I18n.available_locales.each do |l| %>
+  <% enabled_locales.each do |l| %>
     <%= translation_enabled_tag l, site_customization_enable_translation?(l) %>
   <% end %>
   <% contents.each do |group| %>
     <% group.each do |content| %>
       <b><%= content.key %></b>
-      <% content.globalize_locales.each do |locale| %>
+      <% (content.globalize_locales & enabled_locales.map(&:to_sym)).each do |locale| %>
         <%= render Admin::SiteCustomization::InformationTexts::FormFieldComponent.new(content, locale: locale) %>
       <% end %>
     <% end %>

app/components/admin/site_customization/information_texts/form_component.rb

@@ -11,4 +11,8 @@ class Admin::SiteCustomization::InformationTexts::FormComponent < ApplicationCom
     def translation_enabled_tag(locale, enabled)
       hidden_field_tag("enabled_translations[#{locale}]", (enabled ? 1 : 0))
     end
+
+    def enabled_locales
+      Setting.enabled_locales
+    end
 end