Fixed newlines for challenge mails and content title.

Challenge email messages had newlines as code instead of meaningful
characters, that added &13#; to the end of each line. Made sure that
translation system object is forced into a string representation before
running the mail system logic. Code that fetched title for the entity
challenge is attached to was incorrect, there is no meaningful way to
determine the connection as only the field has data about H5PContent
identifier, not the other way around. Changed the code to use the
H5PContent specific title instead.