Render the clone URI input field as readonly.

Summary:
There's no particular reason to allow the user to edit the clone URI field in Diffusion; editing it has no meaning and if you fat finger the keyboard, it's quite possible that the user will either accidentally clear and/or modify the URI before copying (bit me this morning).

Adding a readonly attribute to the input field allows the same benefit (URI is easily selectable) while preventing such accidental input.  Fixes T4246.

Test Plan: Verified that the desired behavior is present in both Chrome, Safari, and Firefox. Field remains selectable with one click, but field is not editable.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4246

Differential Revision: https://secure.phabricator.com/D7810