Use timestamp when issue/comment is created

#65 (Save all issues/comments on install) let postgres save automatically the `created_at` timestamp value. Instead we want this value to be the one when the issue/comment was created to be make sure the history display the content in the correct order

We might need to update the changeset of the issue and comment schema to allow a `created_at` params to be passed on insert. I also need to test if the genereated timestamp by postgres can be overwritten