Split Poll::Booth.search in two methods

We use this method in two different scenarios. In an AJAX request, we
don't want to return every booth if the search is blank. However, in a
normal HTTP GET request, we want to return every record when the search
is empty, as we do everywhere else.

It's possible the behaviour of the AJAX call is unusual, since it
searches all booths, and not just the ones assigned to a poll. If we
changed this behaviour, we could simplify the code and remove the
`quick_search` method.

app/controllers/admin/poll/booth_assignments_controller.rb

@@ -9,7 +9,7 @@ class Admin::Poll::BoothAssignmentsController < Admin::Poll::BaseController
 
   def search_booths
     load_search
-    @booths = ::Poll::Booth.search(@search)
+    @booths = ::Poll::Booth.quick_search(@search)
     respond_to do |format|
       format.js
     end