Allow admins to manage responses to surveys in the admin

Currently, the only way to review responses to a survey is to export them into a CSV-like file. This is uncomfortable in some types of surveys. We would like to add the option to admins to view responses to a survey directly in the admin. Also, as a bonus, we want to allow to export theses responses in a nicely formatted PDF. A PR is created for this in Github, there screenshots there: https://github.com/decidim/decidim/pull/5770