Add template events to viewtopic_body.html

Identifier: viewtopic_body_postrow_post_before
Identifier: viewtopic_body_postrow_post_after
Identifier: viewtopic_body_topic_actions_before (no need to add _after because viewtopic_body_footer_before event is placed there).

Prosilver Location (if applicable): viewtopic_body.html,various places.
Subsilver Location (if applicable): same.

Explanation: these evente allow extensions to add various contents to viewtopic page.
Use case: Most obvious use case is adding advertisements, for example https://www.phpbb.com/kb/article/advertisements-in-phpbb3/ or similar to Advertisement management MOD.