[3.0] End the request when the forum is in maintenance mode

[albertlast]

Description

Maintenance mode does not stop anything. It sets up the notice, sends 503,
and then lets the request it just refused go ahead and run.

Forum::inMaintenance() fills in Utils::$context and returns. preflight()
returns too, and Forum::execute() moves straight on to
self::$current_action->execute().

Two things follow.

The action's side effects all happen. Registering while the forum is in
maintenance creates the member, queues the activation mail, and answers with the
maintenance page — so the only party who does not know the account exists is the
person who just made it:

$ # $maintenance = 1
$ curl -s ... 'index.php?action=signup2' -d 'user=maintuser' ... | grep '<title>'
<title>Maintenance Mode
$ mysql -e "SELECT id_member, member_name FROM smf_members WHERE member_name='maintuser'"
5   maintuser

And what gets drawn is whatever the action left behind. inMaintenance()
sets sub_template, so an action that sets its own wins the page back, and the
board index sets sub_templates and wins it outright. On a forum in maintenance,
as a guest:

Request	Shown
index.php	the board index, every board and last post
?board=1.0	maintenance notice
?action=help	the help page
?action=signup	the registration agreement
?action=stats	maintenance notice

Only the actions that set neither show the notice, which is why this has looked
like it works.

2.1 made this the action rather than a step before one — return 'InMaintenance'
from smf_main() — so nothing else ever ran.

What changes

inMaintenance() ends the request, the same way User::kickIfGuest() does two
checks later in the same method: Utils::obExit() and then die() for the case
that cannot happen.

Testing

With $maintenance = 1, as a guest: every row in the table above is the
maintenance notice with a 503, and ?action=login is still the login form with a
200. As an admin: the forum is untouched, and logging in fresh through the
maintenance form still works. No new rows in smf_log_errors.

Issues References (Fixes|Related|Closes)

Related to #7933