Fixed #13711 -- Long field names on models truncated.

The ticket #13711 and #13528 are somewhat related.
There was a diff given for #13528 by claudep.
I have included those changes in branch as well, and made a single change in it by changing self.model to self.concrete_model.
Then for solving the issue of #13711 I have truncated the field names in the same manner as we do the table names, so that no database error would spring up.
Please review and report if any changes are required or if its fine then well and good.