Fix presentational issues with image diff mode selection.
Review Request #7748 — Created Oct. 31, 2015 and submitted
When initially viewing an image diff, the mode will be set to Two-Up, but that mode won't appear selected in the UI, due to the code setting the class name being nested in a conditional that only executed if this wasn't the first mode set. That has been fixed. When selected, though, the navigation would end up wrapping, due to how the default width was computed. We now compute the width correctly, factoring in padding/margins, to prevent wrapping.
Tested in Chrome and Firefox. Saw that Two-Up was selected by default, and
that it didn't wrap.