Use AphrontFormLayoutView, not AphrontFormView, for GitHub 'Repository' interface

Summary:
See T58. When I originally wrote this interface you couldn't make a form that
looked like a form but wasn't really a form, so I made it a real form. That
meant you could hit "return" in the text input and submit the form, which didn't
do anything but is vaguely silly.

After D422 there's a layout-only form element, so switch to that.

Test Plan: Loaded the tab, hit return in the textarea, it didn't do a form
submit.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason, epriestley
Differential Revision: 719