pools: remove is_active flag.

Remove uses of the is_active flag. Keep column in database (for now).

The only purpose of this flag was to filter out pools from the pool list
in the Add to Pool dialog. This hasn't had much use since autocomplete
was added. Most pools didn't set the flag correctly anyway.

app/controllers/pools_controller.rb

@@ -95,7 +95,7 @@ class PoolsController < ApplicationController
   private
 
   def pool_params
-    permitted_params = %i[name description category is_active post_ids post_ids_string]
+    permitted_params = %i[name description category post_ids post_ids_string]
     params.require(:pool).permit(*permitted_params, post_ids: [])
   end
 end