Implemented Polls for Series

With XF 2.2 taking longer than expected, I am going to move some of the features I've implemented for AMS 2.2 into AMS 2.1.x (starting with AMS 2.1.13).

As per title, I've implemented a Poll Handler for Series. This works just like the Poll Handler for Articles (and every other content type that implements a Poll Handler).

Series has its OWN "Vote on polls" permission (the Vote on polls permission for articles does not come into play with Polls on series pages).



Not going to explain how to add and manage polls as its all the same regardless of content type.

When a Poll is added to a series, it displays on both the "Series parts" page and the "Series details" page (both are new for AMS 2.1.13).