Move Review Board's search to a SearchForm + generic SearchView

Review Request #9105 — Created July 31, 2017 and submitted — Latest diff uploaded

Information

Review Board
release-3.0.x
0227500...

Reviewers

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.