Replace flash helper with "within"

So we can't forget to find "#flash" on the page ever again...

spec/support/flash.rb

@@ -1,16 +0,0 @@
-# frozen_string_literal: true
-
-module Flash
-  def flash
-    results = all(:css, 'div#flash p')
-    if results.empty?
-      return 'none'
-    end
-
-    if results.count > 1
-      texts = results.map { |r| r.text }
-      fail "One flash expected, but we had #{texts.inspect}"
-    end
-    results.first.text
-  end
-end