Add assert_multiple and render_view_and_assert_content

[adam-thomas]

assert_multiple: A helper method to do lots of assert_count() calls at once. Accepts a string, content_to_search, which is the text to be checked, and a dictionary of {content_string: count} items, where 'content_string' is any string, and 'count' is the expected number of instances of that string in content_to_search.

render_view_and_assert_content: Calls access_view_and_render_response then calls assert_multiple on the result. You pass in the {content_string: count} dictionary that assert_multiple uses, and any kwargs used for rendering the view.