In a Single Simple Discussion forum, the first post can only be edited
by a user with the manageactivities capability, but all other posts
behave as normal.
This patch adds new capabilities:
'mod/forum:postprivatereply' - whether a user is able to post private replies; and
'mod/forum:readprivatereplies' - whether a user is able to read private replies.
Private replies are only visible to the intended recipient (the author of
the parent post), the author of the private reply, and those with the
ability to read private replies.
If a post is private then it cannot be replied to further.
