Event - ACP Posting Buttons Before Custom BBCodes

The ACP posting buttons needs the same template event as prosilver has, for right before the custom BBCodes.

Prosilver has: <!-- EVENT posting_editor_buttons_custom_tags_before -->

This does not exist in the ACP, so it's needed so the BBCode buttons will be the same in the ACP as in Prosilver, when extended by an extension.