[ticket/8319] Prepend Board URL to LOCAL_URL links to prevent abuse

The description says:
"The URL must be relative to the topic page and cannot contain a server name
or protocol."
We now enforce this and will add a new token with the current behaviour back.

PHPBB3-8319
[ticket/8319] Add new token RELATIVE_URL to allow foreign relative URL parts

PHPBB3-8319
[ticket/8319] Update BBCodes that currently use the LOCAL_URL tag on update

To fix http://tracker.phpbb.com/browse/PHPBB3-8319 we changed the
second_pass_replace value, so that needs updating for existing ones.

PHPBB3-8319
[ticket/8319] Add explanation for RELATIVE_URL and update LOCAL_URL

The explanation now states that links are prefixed with the board URL.

PHPBB3-8319