Added a link from a diff view to diff's revision if there is one.

Summary:
The diff view page should point to the revision, if the diff has
already been attached to one. The form to select the revision was also
removed in this case.

Let's me know if it should be possible to reattach a diff to a different
revision.

Test Plan:
Tested that a new diff created with '--preview' option was not attached to
any revision. After attaching the diff manually, made sure that the diff view
page
showed the link to the revision correctly.

Reviewed By: epriestley
Reviewers: jungejason, epriestley
CC: aran, epriestley
Differential Revision: 216