refactor: use Phoenix verified routes

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

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

@@ -2,5 +2,5 @@
 ==
 <%= gettext "You recently requested to attend %{title}.", title: @event.title %>
 <%= gettext "You have confirmed your participation. Update your calendar, because you're on the guest list now!" %>
-<%= Routes.page_url(Mobilizon.Web.Endpoint, :event, @event.uuid) %>
+<%= url(~p"/events/#{@event.uuid}") %>
 <%= 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." %>