The can_publish_artefact method on the User class, introduced in
commit aba54873

 (see bug #655631) is out of line with other
permissions on the site when it comes to institution artefacts.

Any institution member can include institution files on one of their
own pages, but this method (currently used when putting images into
forum posts) only allows publishing by institutional admins.

This change adds publishing permission on institution artefacts for
all institution members.

Change-Id: I3f8a15de573de6f58497ae45839647b462fa5e89
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>