Restrict maximum task title length to 255 characters

Summary:
A task title is supposed to summarize a task. For long novels, there is the task description.

Closes T15898

Test Plan: Go to http://phorge.localhost/maniphest/task/edit/form/default/ and enter a task title with multi-byte characters above the threshold, press "Enter".

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15898

Differential Revision: https://we.phorge.it/D25759