fix(rxApp): Have `isActive` compare chunks of URL

This commit also adds an `rx-app-key-{{ item.key }}` class, to help
write selectors for tests. Now that we have two nav menu items
with `.current-search`, some tests were failing that relied on
`$('.current-search')` returning just a single element
Merge pull request #495 from rackerlabs/isActiveChunks

fix(rxApp): Have `isActive` compare chunks of URL