Reword View Requests to Browse Requests in nav

User testing highlighted that it was easy to mistake "View Requests" for
a Facebook-style wall rather than the list of all requests. [1]

[1] https://github.com/mysociety/alaveteli/issues/1505
Use existing "View Requests" key for "Browse Requests"

Also for "View and search requests" --> "Browse and search requests"