Add CONFERENCE_WILL_JOIN action

It's fired before creating the conference with lib-jitsi-meet.

react/features/base/conference/actionTypes.js

  */
 export const CONFERENCE_JOINED = Symbol('CONFERENCE_JOINED');
 
+/**
+ * The type of the Redux action which signals that a specific conference will be
+ * joined.
+ *
+ * {
+ *     type: CONFERENCE_WILL_JOIN,
+ *     room: string
+ * }
+ */
+export const CONFERENCE_WILL_JOIN = Symbol('CONFERENCE_WILL_JOIN');
+
 /**
  * The type of the Redux action which signals that a specific conference has
  * been left.

react/features/base/conference/actions.js

     CONFERENCE_FAILED,
     CONFERENCE_JOINED,
     CONFERENCE_LEFT,
+    CONFERENCE_WILL_JOIN,
     CONFERENCE_WILL_LEAVE,
     LOCK_STATE_CHANGED,
     SET_PASSWORD,
     };
 }
 
+/**
+ * Signals the intention of the application to have the local participant leave
+ * a specific conference. Similar in fashion to CONFERENCE_LEFT. Contrary to it
+ * though, it's not guaranteed because CONFERENCE_LEFT may be triggered by
+ * lib-jitsi-meet and not the application.
+ *
+ * @param {string} room - The JitsiConference instance which will
+ * be left by the local participant.
+ * @returns {{
+ *      type: CONFERENCE_WILL_JOIN,
+ *      room: string
+ *  }}
+ */
+function _conferenceWillJoin(room) {
+    return {
+        type: CONFERENCE_WILL_JOIN,
+        room
+    };
+}
+
 /**
  * Signals that a specific conference has been left.
  *
             throw new Error('Cannot join conference without room name');
         }
 
+        // XXX Lib-jitsi-meet does not accept uppercase letters.
+        const _room = room.toLowerCase();
+
+        dispatch(_conferenceWillJoin(_room));
+
         // TODO Take options from config.
         const conference
             = connection.initJitsiConference(
-
-                    // XXX Lib-jitsi-meet does not accept uppercase letters.
-                    room.toLowerCase(),
+                    _room,
                     { openSctp: true });
 
         _addConferenceListeners(conference, dispatch);