[3.0] Import SMF\Time in Display, so a topic with a reply can be read

[albertlast]

Description

Any topic that has been replied to returns a 500.

Display::loadMessages() measures the gap between consecutive posts to decide
whether to warn about replying to an old topic:

$since = Time::create('@' . $prev_timestamp)->diff(Time::create('@' . $message->poster_time));

SMF\Time is not imported, so inside namespace SMF\Actions that name resolves
to SMF\Actions\Time, which does not exist:

Class "SMF\Actions\Time" not found  —  Sources/Actions/Display.php:335

The surrounding block is guarded by:

if (
	!empty(Config::$modSettings['oldTopicDays'])
	&& (
		empty(Theme::$current->options['view_newest_first'])
			? $message->id > Topic::$info->id_first_msg
			: $message->id < Topic::$info->id_last_msg
	)
) {

oldTopicDays defaults to 120, so the only thing standing between a forum and
this is having no post after the first one in any topic.

Testing

Clean install of release-3.0 (a7ac468b1), MySQL, default settings. The
shipped "Welcome to SMF!" topic reads fine with its single post. Add one reply to
it and:

	?topic=1.0	smf_log_errors
before	HTTP 500, "An error has occurred"	Class "SMF\Actions\Time" not found at Display.php:335
after	HTTP 200, both posts rendered	nothing new

composer lint is clean.

How it was found

By sweeping Sources/ for class names used unqualified inside a sub-namespace
that are neither imported nor declared there — the same shape as the
ThemeChooser bug in #9387. Two of the five candidates that survived filtering
were real; this is the more serious one. The other is a migration and is in a
separate PR.

If it would be useful I am happy to tidy the script up and propose it as a CI
step, since neither phplint nor php-cs-fixer catches this class of mistake and
it fails only at runtime.

Issues References (Fixes|Related|Closes)

Related to #7933