Replace snapshot keyboard navigation with more general solution

In 655263bf12837ef I added code that allows us to use
data-keyboard-focusable attributes to specify which elements on a page
are focusable via keyboard shortcuts j and k, and openable via keyboard
shortcuts o and Enter. This commit completes some of the cleanup
promised there by partially replacing the system we were using for
navigation between snapshots within a sweep with this new, more general
system. To finish this work, I'll be adding keyboard shortcuts for [ and
] which perform a j or k, respectively, immediately followed by an o
keyboard shortcut.

Change-Id: Ia7db2e2beedaef6773c032d46c2c09f948ca263c
Ignore Redis dataset snapshot

In our configuration, Redis creates a dataset snapshot periodically and
saves this snapshot to disk in dump.rdb. We won't even want to commit
this file, so I am adding it to our .gitignore file.

More info:

  http://redis.io/topics/persistence

Change-Id: Iccbc237018e325cc3fa2fb94f49b37dda6a15f9c