Show warning when build queue is backed up

https://github.com/rtfd/readthedocs.org/issues/758

Started taking a pop at this. Have taken a different approach to the linked issue, I've added a context processor
instead of making another request on page load. Are there any obvious issues with this approach? 

It's not yet ready to merge, I could use a hand with the styling, not sure how you want to handle that. I imagine you'll want to tweak the copy too.