Add Translation resource (#495)

Following the API documentation (https://developer.zendesk.com/api-reference/help_center/help-center-api/translations), this will enable CRUD operations for Article / Section / Category translations.

Notes:
- Translation lookups use the `locale`, which needs to be passed in the `id` param to `#find` / `#find!` (ie. `article.translations.find(id: 'fr-FR')`)
- The path for `#destroy` (https://developer.zendesk.com/api-reference/help_center/help-center-api/translations/#delete-translation) differs from the read paths for these objects. Curiously, the default `#destroy` seems to work on Article translations, but not for Section or Category translations. One workaround until this is resolved is to modify the path in a block

```ruby
translation = client.categories.translations.find(id: 'fr-FR')
translation.destroy do |req|
  req.path = "#{client.config.url}/help_center/translations/#{translation.id}"
end
```

Author: codealchemy

lib/zendesk_api/resources.rb

@@ -191,8 +191,10 @@ def resource_path
 
     class Section < Resource
     end
+    class Translation < Resource; end
 
     has_many Section
+    has_many Translation
   end
 
   class Section < ReadResource
@@ -205,10 +207,13 @@ def resource_path
     has Category
 
     class Vote < Resource; end
+    class Translation < Resource; end
     class Article < Resource
       has_many Vote
+      has_many Translation
     end
 
+    has_many Translation
     has_many Article
   end
 
@@ -221,6 +226,8 @@ def resource_path
 
     class Vote < Resource; end
     has_many Vote
+    class Translation < Resource; end
+    has_many Translation
   end
 
   class TopicSubscription < Resource

spec/live/article_spec.rb

@@ -5,6 +5,12 @@
     expect(article).not_to be_nil
   end
 
+  it "can have translations", :vcr do
+    article.translations.create(locale: "fr", title: "Traduction", body: "Bonjour")
+
+    expect(article.translations.map(&:locale)).to include("fr")
+  end
+
   describe "creating articles within a section" do
     def valid_attributes
       { :name => "My Article", user_segment_id: nil, permission_group_id: 2801272, title: "My super article" }

spec/live/category_spec.rb

@@ -5,6 +5,12 @@ def valid_attributes
     { :name => "My Category" }
   end
 
+  it "can have translations", :vcr do
+    category.translations.create(locale: "fr-ca", title: "Traduction", body: "Bon matin")
+
+    expect(category.translations.map(&:locale)).to include("fr-ca")
+  end
+
   it_should_be_creatable
   it_should_be_updatable :position, 2
   it_should_be_deletable

spec/live/section_spec.rb

@@ -5,6 +5,12 @@
     expect(section).not_to be_nil
   end
 
+  it "can have translations", :vcr do
+    section.translations.create(locale: "es", title: "Traducción", body: "Hola")
+
+    expect(section.translations.map(&:locale)).to include("es")
+  end
+
   describe "creating sections withing categories" do
     def valid_attributes
       { :name => "My Section" }