tigefa4u projects Trac

Introduce an !AttachmentContext (partial port of r4595 from the security branch)

Use pseudo-column ''realm'' instead of ''resource'', in TracReports (from the security sandbox r4556)

Some `req.hdf` related clean-ups ported from the security sandbox (r4505 and r4698).

WikiContext improvements: introduce Context subclasses, corresponding to Wiki (r4566) and Ticket (r4569) resources.

Also clean-up trac.wiki.api by removing the old Context from there (now in trac.context).

Backport r4735, the trac.context doctest fixes.

PermissionPolicy: merge with log:trunk@...:4734, plus trac.context doctests fixes

(No more trac.resource, see trac.context instead)

WikiContext improvements: gradual move from the Wiki concern toward a more generic resource descriptor concern.

A Context at this point is still:
- a way to specify ''what'' Trac resource is being accessed. This is the ''resource descriptor'' side, the ''input'' details.
- a way to specify ''how'' that Trac resource is being accessed. This is the ''rendering context'' side, the ''output'' details.

This also brings in some changes done in the [PermissionPolicy security] branch, focusing for this changeset on the r4666 fixes.

Small fix for r4728, needed when `--pidfile` is not specified.

PermissionPolicy: merged log:trunk@...

SpamFilter (captcha): Merged log:sandbox/spam-filter@...:4508