Update routing to use gorilla/mux
Remove unnecessary check for URL path in serveHome

* Using mux renders this check redundant
Update routing so /static/ bypasses mux

* Mux will serve only the top-level directory
* http.Handle allows it to descend recursively as needed
Update /callback so HTTP method type is enforced by mux

* Mux routing can match against HTTP method, so no need to do this
* manually
Create renderTemplate function to abstract template rendering
Use renderTemplate everywhere possible except serveHome

* Leave serveHome as an example of manual template inheritance