New feature: Reopening polls

So happy to be able to put this one in! For the longest time, if you had a poll close before you wanted it to (either you wanted to hear from more people, or things changed and you wanted to revise it to gather more opinions), you had no recourse; we forced you to create a new poll.

This simple feature changes all that! Now if you are a poll author or administrator, you can simply choose 'reopen' from the poll dropdown, set a closing date in the future, and wait for more responses to roll in.

As always, let me know how you get on, and how we can make this better.