This patch adjusts HTML forms processing so that a form
may require properties be set within linked items, not
just in the default item.

Background: my organization has been using Roundup with
a telephony application (Ovolab Phlink), such that each
incoming voicemail message creates a new tracker issue.
 This works beautifully, although we do get our share
of hang-up calls, phone spam and other annoyances.

We've been trying to rework the schema so that an issue
may relate to any number of phone calls and external
e-mails.  And we've wanted to add "assistant" forms to
guide volunteers through triaging these raw inputs into
newly created issues, or connecting them into existing
issues, or discarding them altogether.

These forms had kicked my butt for a year, until I
learned enough Python to realize that I can't use e.g.
"@required=issue-1@title,issue-1@topic,issue-1@@note"
-- only because the cgi/form_parser.py doesn't expect
it.  Although it looks like it was meant to do so
eventually.

This patch tells form_parser to follow links when
checking required properties.  However it's incomplete:
it may not be able to do so 1) beyond a depth of one
link traversal, or 2) if the @required comes before the
@link@ declaration (perhaps a flashback to bug
[740214]).  Within the limits of my familiarity with
Python and Roundup so far, I do think it moves the ball
forward...