backend: add TurboGears2 DebugBar support

DebugBar is a toolbar overlayed over the Kallithea web interface, allowing
you to see:

* timing information of the current request, including profiling information
* request data, including GET data, POST data, cookies, headers and
environment variables
* a list of executed database queries, including timing and result values

Enable it automatically when in debug mode and all requirements are met.
Add documentation.