Refactor Users#upload_avatar method

Moved avatar file upload to ```AvatarUploadService``` class and
```AvatarUploadPolicy```

Address review comments + require missing file in spec

lib/avatar_upload_service.rb

@@ -0,0 +1,43 @@
+class AvatarUploadService
+
+  attr_accessor :source
+  attr_reader :filesize, :file
+
+  def initialize(file, source)
+    @source = source
+    @file , @filesize = construct(file)
+  end
+
+  def construct(file)
+    case source
+    when :url
+      build_from_url(file)
+    when :image
+      [file, File.size(file.tempfile)]
+    end
+  end
+
+  private
+
+  def build_from_url(url)
+    temp = ::UriAdapter.new(url)
+    return temp.build_uploaded_file, temp.file_size
+  end
+
+end
+
+class AvatarUploadPolicy
+
+  def initialize(avatar)
+    @avatar = avatar
+  end
+
+  def max_size_kb
+    SiteSetting.max_image_size_kb * 1024
+  end
+
+  def too_big?
+    @avatar.filesize > max_size_kb
+  end
+
+end