Render an edit link in Diffusion directory views

Summary:
If the user has an editor configured, an Edit link appears next
to the History link.

Somewhat suboptimally, the column is still there if there are no edit
links, it's just empty. I don't know if it matters...

Test Plan: Load Diffusion pages while changing editor setting in preferences.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, avivey

Maniphest Tasks: T1558

Differential Revision: https://secure.phabricator.com/D3124