Fix mobile navigation

One thing that really bugs me about Bootstrap's mobile nav menu is that it doesn't collapse when you tap a link. I just navigated somewhere, and now I want to read it, Twitter Bootstrap!

So this commit adds a JavaScript file containing a function that collapses it when tapped on. 

One other issue I ran into with the mobile nav is that when you jump to a section, the title is hidden because of the offset of the height of the persistent top navbar. So this commit also adds jump anchors that compensate for that. 

One weird thing was that the privacy anchor needed extra compensation. Not sure why.

Lastly, you had coded absolute links (on the detroitwaterproject.org domain) to your assets, so it wasn't possible to develop this locally while adding to those files without making those links relative. So I did that. (See the diff on line 28 of `index.html`, for example.) Not sure if making those relative will break anything on your end, like if you've got some DNS cleverness going on or something. But provided those are all hosted in the same directory, it should be good to go!