Return 404 if user does not exist

Our error reporting tool is registering HTTP 500 errors because a webcrawler is visiting the profile activity of users that do no longer exist, and when rendering the page an Exception is raised. The affected URL is https://*domain*/profiles/*nickname*/activity I suggest returning a 404 instead. For that we just have to change find_by per find_by! here: https://github.com/decidim/decidim/blob/master/decidim-core/app/controllers/decidim/user_activities_controller.rb#L15