[ticket/13205] Add a mark all messages read link to PM folders

PHPBB3-13205
[ticket/13205] Remove superfluous newline

PHPBB3-13205
[ticket/13205] Explicitly cast ids to integers and reformat SQL

PHPBB3-13205
Merge pull request #3058 from naderman/ticket/13205

[ticket/13205] Add a mark all messages read link to PM folders