[ticket/11866] Update some occurances of phpbb_db_ to new Namespace

PHPBB3-11866
[ticket/11866] More namespaces

PHPBB3-11866
[ticket/11866] Remove outdated and broken develop script

PHPBB3-11866
[ticket/11866] Only single backslash in .md files

PHPBB3-11866
Merge pull request #1733 from nickvergessen/ticket/11866

Ticket/11866 Fix some more namespace issues