Move some ruby code out of snapshot `#show` template

I added some percentage calculation to the
app/views/snapshots/show.html.haml template in a recent commit. That
code belongs in a ruby file so I moved it to the model. The model is
still thin, so even if the new methods could be considered helpers I
think it's fine to keep them here for a while.
Move snapshot viewport text to breadcrumb

We're trying to save space on this page so that the diff is "above the
fold" when you view a snapshot. Moving the viewport from the status
block table to the breadcrumb block helps save some space.
Move "when" text to "Snapshot" header

We're trying to save some space on the snapshot show page to make the
diff render more to the top of the page. Moving the created at timestamp
to the header saves a line.
Change Url link text on snapshot page

The link to the page that was snapshotted used to have the page title as
the link text. Since we have the page title right underneath this text
was a bit redundant. I've changed it to the Url address instead.
Replace 'In review' panel with Previous/Next buttons

We're trying to save space on the snapshot show page. This commit
replaces the big "In review" block with a "Previous" and a "Next"
button.
Change keyboard shortcuts for `[` and `]`

These keys used to move up/down once and then auto-click the currently
focused link. Now that we've moved to pager ("Previous"/"Next") buttons
instead of the "In review" panel, these shortcuts didn't really do much.

For now, I've changed to a specific solution (looks for links in a
`.pager` container). In the future we might consider finding a more
general solution to these shortcut keys.