Change the word users for participants on the entire platform

As @xabier explained in the debate on the meta.decidim (https://meta.decidim.org/processes/roadmap/f/219/debates/55) and @carol through the issue of github (https://github.com/decidim/decidim/issues/4551#issuecomment-461886078), to be consistent with this explanation, I think we should stop using the word users on the platform. Places where I've seen it: User groups > Participant groups User settings > Participant setting (all the levels of user settings use users) Search engine -> Search users and not participants Proposal filter by origin -> User groups and not participant groups Sure there are some more. If you know more, we can add it to the comments.