issue,posting: Fix a potential bug about numbering

The value of number field can increase inadvertently by calling of
save() method. To fix that:
* Save() method increases value of number field if only it is null.
* Set the visibility of number field to protected.
issue,posting: Fix constraint violation of number.

Every issues and postings must have unique (project_id, number) pair.
Insert initial data only if in test mode.