return back documentation of DontSearchFileAttachments option

It was deleted in d789bd3986a4ad63a71bd2450c9aad0c7763a7de (rt-4.0.1rc1)
with usage in sbin/rt-setup-fulltext-index, but it is primary use
case is in TicketSQL to filter out named attachments while searching
content.

Mentioned commit replaces use of the option with command line argument.
It was the right call to separate FTS indexing from this option, so
we don't restore usage in the script.
allow class names to change their cases.

we shall mark the new name valid if the duplicated class we found(which was
loaded by name case-insensitively) is the same one that is going to change.

an edge case to note: our old test used the same RT::Class object to try
creating another duplicated class, which breaks new code. but this is very
rare in real world and should be avoided.
Log the subject of mail errors in addition to the explanation

The subject often offers an indication of the action and email that
failed, while the explanation is usually pretty terse.  This is
especially true when the explanation is the direct result of an API
call.
Include Correspond/Comment in the subject when recording a mail error

This indicates for the RT admin looking in the logs what type of action
failed.
allow to set DB's host for tests

allows me to run tests against DB in a VM without trickery
Add Check/Clear All and Reset buttons for the Quick Search (queue list) preferences

Very helpful when the list of queues is long.
Add language and timezone to SelfService preferences
set cache related headers on helper endpoints

* 1-2 min cache for share/html/Helpers/Autocomplete/*
* no cache for share/html/Helpers/*
refactor StaticFileHeaders to use CacheControlExpiresHeaders + test
Install etc/upgrade/ for the benefit of rt-setup-database

Without etc/upgrade/ only some actions that sbin/rt-setup-database
provides are available from an install; the rest need to be run from an
RT tarball or git checkout.  It's more annoying to setup a new RT
instance for an upgrade, test it, and then do the cutover when you can't
run `sbin/rt-setup-database --action upgrade` repeatedly from your
known-good config and install.

We install etc/acl.* and etc/schema.* already, so there's precedent for
installing schema and ACL data files.
Clarify what RT.pm's documentation is about
Convenient initialization via "use RT -init;"

This allows external code to turn the annoying boilerplate of:

    use RT;
    RT->LoadConfig;
    RT->Init;

into

    use RT -init;

as long as it doesn't do anything funky between loading RT, calling
LoadConfig and calling Init.
Merge branch '4.0/helpers-http-cache-headers' into 4.0-trunk
Position the page menu in aileron to avoid overlapping the inner shadow

The extra 0.05em accounts for the border pixels in the elements visually
above the page menu.

Unfortunately, the #page-navigation div is outside of the #body, which
means we can't take the easy route and position relatively up into the
corner.  We're left with approximating the distance from the top of the
viewport to the top of #body in CSS.  This distance is a combination of
element heights, paddings, and margins specified in ems (or 0.9ems for
every 1em when font-size: 0.9em) plus borders specified in pixels.

Resolves [rt3 #16806].
restore background-color transparency for ckeditor

transparency is important here because we customize bg color elsewhere (e.g.
marking it red for ticket reply)
Specify 'no-cache' explicitly for endpoints under /Helpers/

This is the default behaviour of CacheControlExpiresHeaders(), but being
explicit makes it clearer when reading the code.

No functional change.
Merge branch '4.0/fix-ckeditor-bg-color' into 4.0-trunk
Localize "dashboard" word in result of a dashboard creation
Merge branch '4.0/misc-localization-fixes' into 4.0-trunk
Merge branch '4.0/restore-DontSearchFileAttachments' into 4.0-trunk