FEATURE: Change very high/low search priority to rank at absolute ends.

Prior to this change, we had weights for very_high, high, low and
very_low. This means there were 4 weights to tweak and what weights to
use for `very_high/high` and `very_low/low` pair was hard to explain.
This change makes it such that `very_high` search priority will always
ensure that the posts are ranked at the top while `very_low` search
priority will ensure that the posts are ranked at the very bottom.

db/migrate/20201223071241_delete_stale_category_search_priorities_from_site_settings.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class DeleteStaleCategorySearchPrioritiesFromSiteSettings < ActiveRecord::Migration[6.0]
+  def up
+    execute <<~SQL
+    DELETE FROM site_settings WHERE name IN ('category_search_priority_very_low_weight', 'category_search_priority_very_high_weight')
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end