refactor: use Phoenix verified routes

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/web/templates/email/event_participation_approved.text.eex

@@ -6,6 +6,6 @@
 
 <%= gettext "Good news: one of the event organizers just approved your request. Update your calendar, because you're on the guest list now!" %>
 
-<%= Routes.page_url(Mobilizon.Web.Endpoint, :event, @event.uuid) %>
+<%= ~p"/events/#{@event.uuid}" |> url() |> URI.decode() %>
 
 <%= gettext "Would you wish to update or cancel your attendance, simply access the event page through the link above and click on the Attending button." %>