Reword View Requests to Browse Requests in nav

Fixes #1505.

User testing highlighted that it was easy to mistake "View Requests" for
a Facebook-style wall rather than the list of all requests. [1]

I've added a flag to only make this change for WDTK because translations,
but I'm not sure how easy it would be to test that the change is good or not,
so I'm thinking we just roll with "browse"?

[1] https://github.com/mysociety/alaveteli/issues/1505