Fixed the alignment of line numbers on moved rows in diffs.

A couple fixes were attempted in the past to fix alignment of line
numbers whne moved rows came into play. The reason they didn't work was
the (fairly reasonable) assumption that every moved row had the
'.moved-row' CSS class. This was only set for the rows with move flags,
though.

Since that's a pretty decent assumption to make, let's make it true.
Every moved row now has the '.moved-row' class applied.
Fix the style for moved replace lines.

An older style from my initial move enhancement change earlier in 2.0
caused lines to have excess padding where they shouldn't.

The intent, I believe, was to add some padding between the move flag and
the text, but why this was only on replace lines, I'm not sure. This
change fixes the style by marking lines where a move flag is shown, and
adding 4x of padding between it and the text on both columns.
Show and hide the "Download Diff" link depending on the interdiff state.

When I made the diff revision selector operate without reloading the page, I
neglected the "Download Diff" link at the top of the page. This link should
only be shown when the page is displaying a full revision, and not when there's
an interdiff.

I've changed it to use an inline style to hide the link on the first page load
if appropriate, and JS to show/hide it when the displayed revision changes.

Testing done:
Played around with the revision selector and watched the link show and hide
itself. Verified that if I initially loaded an interdiff, that the link was
present in the DOM to show later.

Fixes bug 3269.

Reviewed at https://reviews.reviewboard.org/r/5560/