Add date from message when creating a post from it, fixes T253

Summary:
  - Added created value from message model when creating a post from a message

Test Plan:
  - Go to messages
  - Create a post from a message
  - Check post table to see `created` value from message populated in it.

Reviewers: rjmackay, shadowhand

Reviewed By: shadowhand

Subscribers: srutto, lkamau

Maniphest Tasks: T253

Differential Revision: https://phabricator.ushahidi.com/D290