models: constraint naming fix
models: constraint naming fix

* Fixes issue with auto-generated constraint name being too long for
  MySQL.

Signed-off-by: Lars Holm Nielsen <7eaffca9f7cd98839b5821d0d23e09ff764b82cd@cern.ch>

tiborsimko/invenio-oauthclient,inveniosoftware/invenio-oauthclient,tiborsimko/invenio-oauthclient,inveniosoftware/invenio-oauthclient,tiborsimko/invenio-oauthclient,inveniosoftware/invenio-oauthclient