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

PHPBB3-13138
[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/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
Merge pull request #3039 from nickvergessen/ticket/13138

[ticket/13138] Only use cookie data when we do not force a user_id
Merge branch 'develop-olympus' into develop-ascraeus

* develop-olympus:
  [ticket/13138] Do not use cookie data and autologin while forcing a user_id

Conflicts:
	phpBB/phpbb/session.php