feat(back): add admin setting to disable external event feature

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

config/config.exs

@@ -41,7 +41,10 @@ config :mobilizon, :instance,
   email_reply_to: "noreply@localhost"
 
 config :mobilizon, :groups, enabled: true
-config :mobilizon, :events, creation: true
+
+config :mobilizon, :events,
+  creation: true,
+  external: true
 
 config :mobilizon, :restrictions, only_admin_can_create_groups: false
 config :mobilizon, :restrictions, only_groups_can_create_events: false