Checks whether the current user can link an event to the current topic.

• First checks to see if you can even post calendar events
• Then checks to see if the topic and board have been set ($topic and $board global variables)
• Lastly checks to see if you're an administrator, moderator or owner of the topic
• Dies with a fatal error if you can't post calendar events (via isAllowedTo()), or a fatal_lang_error if any other condition isn't met