INTEG-216: [integ-forum] i18n for the message of number of replies in Topic Activity

Fix description:
* Define new keys for that message depending on the number value
* Use these keys to get the appropriate localized message

Conflicts:

	integ-forum/integ-forum-social/src/main/java/org/exoplatform/forum/ext/impl/ForumUIActivity.java
	integ-forum/integ-forum-social/src/main/resources/locale/forum/integration/integration_en.properties