[ticket/10308] disable retain/ delete posts option when deleting a user

When deleting a user, it asks whether the posts by user should be retained
or deleted. The selection should be disable if the user has no posts.

PHPBB3-10308
[ticket/10308] Displays message to user if there are no posts.

While deletng the user, if the user has no posts in addition to the
options being disabled an added message is displayed.

PHPBB3-10308
[ticket/10308] fixes language entity.

Language key has been changed and has been made more specific to
avoid conflicts

PHPBB3-10308
[ticket/10308] fix language and user's total posts

language modified to be clear and select box disappears in case no
posts by user. user's total posts are fetched using a new query.

PHPBB3-10308
[ticket/10308] renames language key to USER_NO_POSTS_DELETE

language key renamed to make its usability more clearer.

PHPBB3-10308
[ticket/10308] fixes user deletion if no posts

introduces a hidden input field with retain posts as the mode in case
user has no posts.

PHPBB3-10308
[ticket/10308] fixes sql query, limit it to 1

instead of fetching all posts by user we limit the query to 1 to check if
a user has posts or not

PHPBB3-10308
[ticket/10308] makes variable boolean

makes user_row['user_has_posts'] boolean instead of 1 or 0.

PHPBB3-10308
[ticket/10308] fixes language variable name

Language variable has be renamed for better understanding

PHPBB3-10308
[ticket/10308] fixes indentation

indentation is fixed and user_posts variable is compared as a boolean
variable.

PHPBB3-10308
[ticket/10790] Remove a (kind of) dead code section from submit_pm().

The type of $user->data['user_id'] is (almost) guranteed to be integer
since session::session_create() casts it to integer.
The type of $row['user_id'] is only an integer if the DB driver adjusts the
PHP type according to the DB type. This is only done by some of our
not-so-popular DB drivers and is not the case for MySQL.

As such this comparison is (almost) never true and a PM is also sent to the
author itself when it is sent to a group the author is also a member of.

Since this behaviour seems to have been accepted by the communty, the dead
code is removed and current behaviour is kept.

Also, checking this in the loop seems to be a rather bad idea.

Introduced by 78b1c4caaa17cc8760b685ad41c19f15f9d89b68.

PHPBB3-10790
[ticket/10912] Default last post subject to empty lacking last post info

PHPBB3-10912
[ticket/10909] Also test develop-olympus with low PHP 5.3 version on travis.

Also test develop-olympus with low PHP 5.3 version (i.e. PHP 5.3.3) on travis.

PHPBB3-10909
Merge branch 'ticket/10909' into develop-olympus

* ticket/10909:
  [ticket/10909] Also test develop-olympus with low PHP 5.3 version on travis.
Merge branch 'develop-olympus' into develop

* develop-olympus:
  [ticket/10909] Also test develop-olympus with low PHP 5.3 version on travis.

Conflicts:
	.travis.yml
[ticket/10908] Download files only up to max_upload_filesize if limit is 0

PHPBB3-10908
[ticket/10913] Redirect to index if session id is required but was not sent

PHPBB3-10913
[ticket/10550] Sort not installed styles list in the styles section of the ACP.

PHPBB3-10550
Merge remote-tracking branch 'imkingdavid/ticket/10912' into develop

* imkingdavid/ticket/10912:
  [ticket/10912] Default last post subject to empty lacking last post info
[ticket/10908] Document that 0 filesize configuration means limited by PHP

PHPBB3-10908