INTEG-322: [integ-forum] Display the total number of voters on Poll Activity

Fix description:
* Show the number of voters instead of the number of votes.
INTEG-306: Wrong display activity after merging topic

Fix description:
* Update method updateActivityByTopic in ForumActivityUtils class to create new comment when comment associated to post doesn't exist.
* Use ForumActivityUtils.saveTopicActivity instead of ActivityManager.saveActivityNoReturn when merging topic.

Conflicts:
	integ-forum/integ-forum-social/src/test/java/org/exoplatform/forum/ext/forum/ForumActivityTestCase.java