Hovercards - don't let them run off the right edge of the viewport anymore

Summary: Fixes T7524.

Test Plan:

• made a task with a comment including another task. resized window so still desktop size and task reference on edge of window. invoked hovercard by bovering over task reference and noted the hovercard was completely visible.
• opened the durable column and made a task reference. invoked hovercard by hovering over task reference and noted the hovercard was completely visible.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7524

Differential Revision: https://secure.phabricator.com/D12759