Add time estimates to all the challenges and sections.

[lindsay-fredrick]

I realized I haven't been doing this but we should go back through and add time estimates for all of the challenges at some point (at the very least, it will be interesting to see how good we are at guessing how long we should be giving!)

The sections should probably get estimated as well, but I think we'll get a better idea of how long each section should be once we have more put together.

[swayneh]

sounds good