This patch addresses this RFE by adding a 'mute' parameter to
roundupdb.IssueClass.nosymessage that omits the change note.

It allows one to omit non-journaled *Link properties from an issue
change note by setting a 'quiet' parameter in nosymessage() that reaches
generate[Create|Change]Note().

It also addresses http://psf.upfronthosting.co.za/roundup/meta/issue249
and this RFE by allowing one to hide any non-*Link properties by setting
an attribute 'do_journal=False' on them. This affects both change notes
and history (in templating).

Example usage in schema.py:
nosy_count = Number()
nosy_count.do_journal = False
issue = IssueClass(db, "issue",
                   type=Link('issue_type'),
                   [...]
                   nosy_count=nosy_count)