[ticket/8960] Allow changing avatar path if it's not writable

This change will allow users to change the avatar path even if it's not
writable. However, this will get rid of the error message that the path is
not writable. Due to this, a minor addition to the explain message in the ACP
settings has been made.

PHPBB3-8960
Merge branch 'ticket/8960-olympus' into ticket/8960

Conflicts:
	phpBB/includes/acp/acp_board.php
[ticket/8558] Add display name in emails from board

PHPBB3-8558
[ticket/8558] Do string concatenation only once and do htmlspecialchars

PHPBB3-8558
[ticket/12404] Adds a new event before and after the Timezone display

PHPBB3-12404
[ticket/8558] Add display name in emails from board

PHPBB3-8558
[ticket/12409] Add acp_users.php core events to modify users preferences data

There're events to modify UCP user preferences in ucp_prefs.php,
so it makes sense to add related events to acp_users.php to control
new user's prefs via ACP.

PHPBB3-12409
[ticket/8558] Add config in migrations and schema_data

PHPBB3-8558
[ticket/8558] Use only one variable and improve explain string

PHPBB3-8558
[ticket/8558] Fix field length in ACP

PHPBB3-8558
[ticket/12273] Add a basic set of tests for the exporter

PHPBB3-12273
[ticket/12273] Use the new class

PHPBB3-12273
[ticket/12273] Grab name of events with a function

PHPBB3-12273
[ticket/12273] Move grabbing the $vars array line to a method

PHPBB3-12273
[ticket/12273] Add more tests for the event exporter

PHPBB3-12273
[ticket/12273] Use class properties instead of parameters

PHPBB3-12273
[ticket/12273] Crawl the phpBB directory for events

PHPBB3-12273
[ticket/12273] Move event exporter to namespace

PHPBB3-12273
[ticket/12273] Rename exporter to php_exporter

PHPBB3-12273
[ticket/12273] Move MD Exporter to separate file

PHPBB3-12273