INTEG-230 | Mention Name in activity stream is not shown correctly

Fix description: Update method saveToNewDataFormat in UIDocActivityBuilder class. So mention processor works correctly with activity context.
INTEG-242 | [integ-forum][Migration] Some problems of Forum Activity after migration from PLF 3.5

Fix description: When we can not build a post from an old activity, we create a new one and set property for this post.