[ticket/11777] Add subdirectory 'events/' to the template event search path

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
[ticket/11777] Move the testing template events to 'events/' subdirectories

The tests written for extension template events did not follow the convention
and documentation of placing template event files in the events/ subdirectory.
Moved the files to this subdirectory so the tests succeed again.

PHPBB3-11777
[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
[ticket/11777] Fix new test for loop variables in extension template listeners

With the merge of https://github.com/phpbb/phpbb3/pull/1564 a new test has
been added. Renamed and moved the template listener file of that test to
comply with the new requirements.

PHPBB3-11777
Merge pull request #1643 from rechosen/ticket/11777

[ticket/11777] Search for template listeners in the php event listeners way