Enable participate feature with user-created activity.

<How to verify>

1. First, login with Discord account, so you can create acitivity.
2. Create an activity. (The just created activity will have `signup` property as `0` since this commit.)
   After creation succeeds, you can see the new actvity showing up in the main page.
3. At the main page, click the new activity, and click `participte`.

   => you will get something like
   `successfully signup` if you do it the first time.
   or
   `you have signed up the session: $title` if you do it multiple time.

src/co/gaiwan/compass/routes/sessions.clj

@@ -39,6 +39,7 @@
              :session/description description
              :session/type (keyword "session.type" type)
              :session/location (keyword "location.type" location)
+             :session/signup 0
              :session/capacity (parse-long capacity)}
       (= ticket-required? "on")
       (assoc :session/ticket-required? true)