[ticket/13138] Do not use cookie data and autologin while forcing a user_id

PHPBB3-13138
Merge branch 'ticket/13138-2' into ticket/13138-asc

Conflicts:
	phpBB/phpbb/session.php
[ticket/12568] Replace modifications explanation with extensions.

PHPBB3-12568
[ticket/12568] Update path to language pack installation page.

PHPBB3-12568
[ticket/12568] Update style installation instructions.

PHPBB3-12568
[ticket/12568] Remove various unnecessary references to phpBB3.

PHPBB3-12568
[ticket/12568] Fix typo - "directorie" -> "directories"

PHPBB3-12568
[ticket/12568] Update GitHub link.

PHPBB3-12568
[ticket/12568] The "Purge cache" option in the Customise tab no longer exists.

PHPBB3-12568
[ticket/12568] Fix awkward use of "phpBB Limited."

PHPBB3-12568
[ticket/13138] Fix the session key tests

PHPBB3-13138
[ticket/13138] Type hint the variables for IDE usage

PHPBB3-13138
[ticket/13186] Don't show link on post count when search disabled

PHPBB3-13186
Merge pull request #3045 from Elsensee/ticket/13186

[ticket/13186] Don't show link on post count when search disabled
Merge pull request #2960 from prototech/ticket/12568

[ticket/12568] Update user docs
[ticket/13187] Fix HTML validity of INSTALL.html

PHPBB3-13187
Merge pull request #3046 from Elsensee/ticket/13187

[ticket/13187] Fix HTML validity of INSTALL.html
[ticket/13168] Fix mbstring warnings in ACP for PHP 5.6 compatibility

PHPBB3-13168
[ticket/13188] Add missing where clause to post_id query in sphinx index

PHPBB3-13188
[ticket/13188] Correctly format string concatenation

PHPBB3-13188