Use datetime with TZ for posts timestamps

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/graphql/schema/post.ex

@@ -17,8 +17,8 @@ defmodule Mobilizon.GraphQL.Schema.PostType do
     field(:attributed_to, :actor, description: "The post's group")
     field(:visibility, :post_visibility, description: "The post's visibility")
     field(:publish_at, :datetime, description: "When the post was published")
-    field(:inserted_at, :naive_datetime, description: "The post's creation date")
-    field(:updated_at, :naive_datetime, description: "The post's last update date")
+    field(:inserted_at, :datetime, description: "The post's creation date")
+    field(:updated_at, :datetime, description: "The post's last update date")
 
     field(:tags, list_of(:tag),
       resolve: &Tag.list_tags_for_post/3,

lib/mobilizon/posts/post.ex

@@ -61,7 +61,7 @@ defmodule Mobilizon.Posts.Post do
     belongs_to(:picture, Picture, on_replace: :update)
     many_to_many(:tags, Tag, join_through: "posts_tags", on_replace: :delete)
 
-    timestamps()
+    timestamps(type: :utc_datetime)
   end
 
   @required_attrs [