This will be version 4.0 to match the RT version it is compatible with.

Going forward, we can keep the major version in lockstep.
Update override of RT::Interface::Web::ShowRequestedPage()

This function is overridden to gut the core Self Service handling
because it interferes with our own public-user handling.  Luckily, the
function doesn't change much as evidence by the single-line change
needed to bring it up to parity with RT 4.0.10.
Expand on the comments about RT::Config::META overrides
Remove horrible hack which set WebBaseURL and WebURL for each request

This was to support much older versions of RT when visitors could toggle
between HTTP and a self-signed SSL cert for HTTPS.

Nowadays, we force SSL with a real cert.  This file should have been
removed in dd1507e.
No description is better than one that refers to removed code
Remove custom themes and simply provide a custom style for all

This is what the themes were effectively doing.
Replace massive overlays by simply removing Commenting from the UI

Removes Comment from the Actions menu, "private" from available
UpdateTypes, and the per-transaction "Comment" links.

RT::Ticket->Comment is already overridden to forbid comments at the API
level.
Preserve inc/.author across git clean
Custom 404 handling is a holdover from the RT 3.6 era
Update the version info in About
Update 3.6-era $RT:: style to modern RT-> calls
Update disabling of the Quick Search prefs

While we're at it, remove all links to that prefs page so that most
people won't see the "Sorry, you can't" message.
Update M::I and build framework
Remove support for pre-2006 public dist/bug pages

Nothing should be hitting these URLs, and if they are, it's time to 404
and get 'em fixed.
Custom Logo handling is unnecessary in RT 4
Remove "quick search" and queue summary overlays in favor of callback

Preserves the removal of the link to /Prefs/Quicksearch.html via CSS.

The LinkBuilders callbacks require the 4.0/queue-summary-link-callbacks
RT branch, and preserve the custom links to /Dist/Display.html instead
of /Search/Results.html.  Without the callbacks, we'll need to overlay
the entire files.
Replace custom SelectOwner with the $AutocompleteOwnersForSearch option

The option forces autocomplete for any cases which would generate a huge
dropdown.  The upshot of this is that when editing individual tickets,
a nice dropdown is available!
Keep most of rt.cpan.org's config in this extension

Now only site-specific values like passwords and database hosts can be
in a private RT_SiteConfig.pm.
Replace SelectQueue with an autocompleting text input
Allow editing of quicksearch preferences again, thanks to an RT patch

The patch is from the RT branch 4.0/quicksearch-prefs-callback.