Check CAN_VIEW and CAN_EDIT at SearchAttachController

Summary: Fixes T11193. Assume this is the correct place to check for permissions before attaching edges.

Test Plan: Create a task and set edit policy to Admins, log into test account. Try to Edit Subtasks, Merge Duplicates, Attach a Diff, or Attach a Mock, get a Policy Dialog explaing why.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11193

Differential Revision: https://secure.phabricator.com/D16161