Add follow button on results

Related to this : https://meta.decidim.org/processes/roadmap/f/122/proposals/12305 As a user I want to be able to follow a result and receive notification when it's updated. Notification events : - New comment - Change in percentage of completion - New step added There is one complex thing to handle : how to handle subscription with the parent - child - grand child data structure result have. Example : - if I suscribe to result that has n childs do I receive notifications aout all childs as well ? - if I suscribe to a child result do I receive notifications about the parent ?