comment_model: implement create_qcomment and create_acomment

[jonassanoj]

line 52: Implement create_qcomment and create_acomment so that it adds a question or answer comment to the database and returns the new comment's commentID. Then create its documentation, using phpdoc comments. See get_acomments for an example.

[tilman-schieber]

body is not an int but a string. otherwise good documentation