Handle when a channel which contains a poll is deleted

[cameronkinsella]

Expected Behavior

The poll should be automatically removed from the database

Actual Behavior

1. An exception will be thrown if the deadline for the poll is reached
2. The link provided from the poll list command will be invalid

There are likely other unwanted behaviors as well.

Steps to Reproduce the Problem

1. Create a poll in a channel
2. Delete that channel

[cameronkinsella]

This is now much more likely to occur with the addition of threads.