JOINDIN-443: Add comment source to template

Adds the comment source to the template via a new entity method.
Adds in unit tests to cover the EvenCommentEntity methods.
Updates the existing EventCommentEntity methods to return null if not set.
JOINDIN-443: Add source to talk comments
Simplify control flow of entity get functions
Correct TalkEntity constructor comment
Merge branch 'pr105'
JOINDIN-443 #close Fixed.