[ticket/13023] Add template event posting_editor_buttons_custom_tags_before


[PHPBB3-13023](https://tracker.phpbb.com/browse/PHPBB3-13023)
I'm not sure if I agree with this spot as it's inside the bbcode buttons div. This is mainly cause posting_editor_buttons_after is weirdly placed in comparison between prosilver and subsilver2. This will also cause it to be differently place in relation to that existing event in prosilver vs. subsilver2.