Prevent workboard cards from being grabbed by the "Txxx" object name text

Summary:
Fixes T13452. We currently give users mixed signals about the interaction mode of this text: the cursor says "text" but the behavior is "grab".

Make the behavior "text" to align with the cursor. An alternate variation of this change is to remove the cursor, but this is preferable if it doesn't cause problems, since copying the task ID is at least somewhat useful.

Test Plan: In Safari, Firefox, and Chrome: selected and copied object names from workboard cards; and dragged workboard cards by other parts of their UI.

Maniphest Tasks: T13452

Differential Revision: https://secure.phabricator.com/D20898