[ticket/13302] Update documentation and support forum links to 3.1 versions.

PHPBB3-13302
[ticket/13302] Update translations forum link to 3.1.

PHPBB3-13302
[ticket/13302] Fix incorrect link to extensions database.

PHPBB3-13302
Merge pull request #3225 from prototech/ticket/13302

[ticket/13302] Fix incorrect links to phpBB.com resources