Dumb-down avatar presence check in avatar_url methods

`avatar.present?` goes through CarrierWave, and checks that the file
exists on disk and checks its filesize. Because we're hitting the disk,
this adds extra overhead to something where the worst-case scenario is
rendering a broken image.

Instead, we now just check that the _database attribute_ is present,
which is good enough for our purposes.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/19273

app/models/group.rb

@@ -90,7 +90,7 @@ class Group < Namespace
   end
 
   def avatar_url(size = nil)
-    if avatar.present?
+    if self[:avatar].present?
       [gitlab_config.url, avatar.url].join
     end
   end

app/models/project.rb

@@ -701,7 +701,7 @@ class Project < ActiveRecord::Base
   end
 
   def avatar_url
-    if avatar.present?
+    if self[:avatar].present?
       [gitlab_config.url, avatar.url].join
     elsif avatar_in_git
       Gitlab::Routing.url_helpers.namespace_project_avatar_url(namespace, self)

app/models/user.rb

@@ -653,7 +653,7 @@ class User < ActiveRecord::Base
   end
 
   def avatar_url(size = nil, scale = 2)
-    if avatar.present?
+    if self[:avatar].present?
       [gitlab_config.url, avatar.url].join
     else
       GravatarService.new.execute(email, size, scale)