tigefa4u projects Trac

Fixed the ''RSS Feed'' for tickets, which was broken since the modification of the `Ticket.get_changelog` method.

By doing that, I factored out the grouping of related ticket fields changes
in a new `TicketModule.grouped_changelog_entries` method, which is used
by `TicketModule.export_css` and `TicketModule._insert_ticket_data`.

As a result, the ''RSS Feed'' for tickets, also features links to the precise
change entries within the ticket.

Added attachment filtering, based on `IAttachmentManipulator` interface added
in r3399. Also check `author` field for spam.

Minor method name tweak.

Ticket and Wiki attachments now show up in the timeline.

* Added attachment change listener and validation extension points.
* Added exceptions specific to each of the wiki, ticket and attachment validators.
* Wiki manipulator now actually walks the page validator return values correctly.

Improve `title` of ticket timeline event entries in RSS feeds. Fixes #2070.

The `wiki_to_*` functions now always return a `Markup` object.

Stylistic change: ticket status is now more prominent. Closes #1990.

Timeline ticket events now link precisely to the corresponding ticket change.

Finalizes #3259, proposed by athomas.
I think this could also help for #2760 w.r.t to tickets.

Some more ticket ''Reply to'' feature tweaks:
* added the ability to reply to the main ticket description.
This is mainly interesting for citing part of the original description.
* some Trac developers were not that found of the auto-hide ''Reply'' button,
so I've changed that to a normal button, as less obtrusive as I could make it.