This patch updates Review Board's RBSearchView, which was previously

using Haystack's deprecated SearchView with the generic version added

in 2.4 that builds on top of existing Django forms infrastructure. This

means that we now require a minimum version of 2.4.0 for Haystack.
The actual search code has been split out into a form that does the

actual searching. This will allow searches to be performed without

setting up a bunch of view rendering infrastructure that we won't need

each time (e.g., from the API).

Ran unit tests.