[ticket/11777] Rename the extension template listener subdirectory to 'event/'

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
[ticket/11777] Require a suffix of '_listener' on extension template listeners

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