Add `BookmarksController#show`

Ultimately I want the ability to find out whether I've bookmarked a specific
URL so that I can build a Chrome extension. I'm imagining something like
/bookmarks/<url-of-bookmark>.json but I think this is a good first step.
Use the bookmarked url as the bookmark identifier

This should make it easy to construct a request to check for the existence of
a URL in javascript, e.g. `'/bookmarks/' +
encodeURIComponent(document.location.href)`.

The controller is a bit messy after this commit as I wanted to continue
supporting the viewing of bookmarks by id.

I'm not exactly sold on this URL structure as it feels a bit clumsy/messy but
it'll do for now.