API: refactored last fix, project limit in web client is fixed too

The previous call `saved?` is restored in the `POST /projects` method in the API.
It is refactored to check if the record is persisted. This is useful to not validate
the record again after saving. This fixes the returned status code in the web client
too. If the last project is created via web client instead of error notification
the project page is shown.

app/models/project.rb

@@ -146,7 +146,7 @@ class Project < ActiveRecord::Base
   end
   def saved?
-    id && valid?
+    id && persisted?
   end
   def import?

lib/api/projects.rb

@@ -44,7 +44,7 @@ module Gitlab
                                     :merge_requests_enabled,
                                     :wiki_enabled]
         @project = ::Projects::CreateContext.new(current_user, attrs).execute
-        if @project.persisted?
+        if @project.saved?
           present @project, with: Entities::Project
         else
           not_found!