To further mirror the file name and location requirements for php template
event listeners, require extension template event listener files to follow the
'<event name>_listener.html' naming format.
PHPBB3-11777
Per suggestion of EXreaction and nickvergessen, do not look for extension
template event listeners in styles/[style]/template/events/ but in
styles/[style]/template/event/ (without the trailing 's') to match the way
phpBB looks for php template event listeners.
PHPBB3-11777
Makes the twig template engine look in the events/ subdirectory instead of the
main styles/[style]/template/ directory for extension template events. Note
that it does _not_ look recursively!
PHPBB3-11777
