schema doc update

[Nick Barnes]

[also sent to webtools]

I have updated my online Bugzilla schema documentation to reflect the
recent releases.  The only schema changes are the additions of
flagtypes.grant_group_id and flagtypes.request_group_id in 2.19.2.
The 2.18 schema is the same as 2.18rc3; the 2.16.8 schema is the same
as 2.16.7.

<http://www.ravenbrook.com/project/p4dti/tool/cgi/bugzilla-schema/>

Nick Barnes
P4DTI Project
Ravenbrook Limited