Bug 1631795: Made image in a wallpost public

When an image is added in a wall post it will be also added
in the artefact_file_embedded so it can be seen by other users
Includes an upgrade to update old posts with embedded images.

behatnotneeded

Change-Id: I42d35e035f8fe8c846ef345709793f4c0f66ca7b
(cherry picked from commit d598dc1b)