validate assignment titles

fixes CNVS-10160

test plan
 - attempt to save assignment with title over 255 char
 - should fail and not generate db error

Change-Id: If5633aea38fb10d71367900af841b3ed070c1085
Reviewed-on: https://gerrit.instructure.com/35664
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>