Move gravatar url compose to separate service

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/services/gravatar_service.rb

+class GravatarService
+  def execute(email, size = nil)
+    if gravatar_config.enabled && email.present?
+      size = 40 if size.nil? || size <= 0
+
+      sprintf gravatar_url,
+        hash: Digest::MD5.hexdigest(email.strip.downcase),
+        size: size,
+        email: email.strip
+    end
+  end
+
+  def gitlab_config
+    Gitlab.config.gitlab
+  end
+
+  def gravatar_config
+    Gitlab.config.gravatar
+  end
+
+  def gravatar_url
+    if gitlab_config.https
+      gravatar_config.ssl_url
+    else
+      gravatar_config.plain_url
+    end
+  end
+end