[ticket/12376] Add template events viewtopic_body_polls

I propose 3 template events:

1: `EVENT viewtopic_body_poll_question_prepend`

It would be the equivalent of `viewtopic_topic_title_prepend`, and will allow us to use prefixes, or better yet, icons. It will allow us to use code like:
`<h2><span class="<!-- IF S_CAN_VOTE and L_POLL_LENGTH -->poll-icon-timer<!-- ELSEIF S_CAN_VOTE -->poll-icon-open<!-- ELSE -->poll-icon-closed<!-- ENDIF -->">{L_VIEW_TOPIC_POLL}</span>....</h2>`

2: `EVENT viewtopic_body_poll_fieldset_prepend`
Having control of the poll fieldset can have advantages in case we want to add extra functionality, buttons or create wrapping containers.

3: `EVENT viewtopic_body_poll_fieldset_prepend`
The counter-part to the previous one

PHPBB3-12376