conferences: constrain title length

fixes CNVS-11010

test plan:
  * open a course conferences page
  * open the new conference dialog
  * attempt to enter more than 255 characters in the title field
  * verify that you cannot do so
  * save your new conference
  * verify that it saves successfully

Change-Id: I3d7c5d51ee0c8e224827a576982690cfebc38553
Reviewed-on: https://gerrit.instructure.com/31690
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>