Hide tags and rename "topics"

In #743 @nbradbury pointed out that our browsing experience isn't good enough, so we're hiding the tags in the Reader list for now. Generally I like to avoid commented out code, but the idea will be to reenable this in the following release (added #755 for this).

I also took the opportunity to rename the controller from "Topics" to "Browse" (since it only shows lists by default), and if you've followed tags at WordPress.com, they'll show up under "Tags" instead of "Topics".