[feature/prune-users] Rework user_delete() functions_user.php

user_delete now uses fewer queries to delete a set of users of size > 1 by
accepting an array of users rather than a single user at a time.  This required
changing the third parameter, however the function retains its former behavior
with the old-style parameters.

PHPBB3-9622
[feature/prune-users] Apply e6ed55a9c1ceb07ab2e87d4a53f9e688fda319c5.

This was done in PHPBB3-9872.

PHPBB3-9622
[feature/prune-users] Adjust some language strings for new features

Adjust two language strings for ACP prune feature to include descriptions for
new features.

PHPBB3-9622
[feature/prune-users] Fixed whitespace in language file.

PHPBB3-9622
[feature/prune-users] Call sql_is_set for user ids once in user_delete.

PHPBB3-9622
[feature/prune-users] Replaced missed occurrences of $user_id with $user_ids.

PHPBB3-9622
[feature/prune-users] Call sql_is_set once for author ids in user_delete.

PHPBB3-9622
[feature/prune-users] Fixed user id check in undelivered users loop.

There is now $user_ids instead of one $user_id.

PHPBB3-9622
[feature/prune_users] Moved set_config_count out of user loop.

PHPBB3-9622
[feature/prune-users] Minor UI adjustment and bug fixes

Removed unneeded hidden fields in confirmation page.  Do not show group prune
dropdown if there are no groups, etc.

PHPBB3-9622
[ticket/10605] Check for orphan privmsgs when deleting a user

Also moved the hole code into a new function.

PHPBB3-10605
[ticket/9089] Add tabindex to PM recipient box, to allow tabbing to the subject

I also added tabindex 1 to the buttons for adding the recipients.
Also note, that duplicated tabindex are fine:
http://www.w3.org/TR/html4/interact/forms.html#adef-tabindex

PHPBB3-9089
[ticket/10605] Delete orphan private messages on update

PHPBB3-10605
[ticket/9089] Add tabindex to pm/topic/post icon-options aswell

PHPBB3-9089
[ticket/10605] Prefix function with phpbb_ and use true instead of 1

PHPBB3-10605
[feature/new-tz-handling] Wrapper around DateTime for new date time handling.

Wrapped PHP's DateTime with some extensions for supporting phpBB's relative
date formats and provided the DateTime::getTimestamp() method to PHP < 5.3.

PHPBB3-9558
[feature/new-tz-handling] Use phpbb_datetime rather than phpbb_DateTime.

PHPBB3-9558
[feature/new-tz-handling] Renamed old variables and removed extra conditional.

PHPBB3-9558
[feature/new-tz-handling] Update user methods to use new date processing class.

user::setup() now stores a DateTimeZone object in user::$timezone representing
the users timezone. For backwards compatibility a numeric value in
user/board_timezone will be converted into one of the legacy Etc/GMT±X
timezones. This will be used until the user updates his/her timezone in the
UCP.

user::format_date() is now basically a legacy wrapper that transforms a UTC
UNIX timestamp into a formatted localised date using phpbb_datetime::format().

PHPBB3-9558
[feature/new-tz-handling] Correct capitalisation of phpbb_datetime.

PHPBB3-9558