Some of the recent template and style work made bits of the diff viewer

look a bit ugly. The issue summary table was always present (but empty),

and the diff entries being loaded had a white background behind the

spinner.
This change makes the issue summary table's display conditional,

enabling it only for the review request page. It also changes the look

of the diff entries to have the spinner before the filename, creating a

nice visual consistency and making the entries look nice and slim.
It also moves some padding from .sidebyside.loading into its tbody,

to avoid an issue with the wrong color appearing along the edges.

Viewed the diff viewer page and saw that the issue summary table was no

longer there, and that the diff placeholder entries looked a lot nicer.
Made sure that the issue summary table was visible on the main review

request page.