Make "Create New Similar Task" inherit priorities

Summary: Request in T1589, seems reasonable to me.

Test Plan: Created a "Wishlist" task, created a "Similar" task, got "Wishlist" default priority.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1589

Differential Revision: https://secure.phabricator.com/D3155