Bug 913911 - add a webservice method to expose a bug graphs (including dependency and duplicates)

[dylanwh]

This adds a graph API that can return a graph of bugs based on either the dependencies table or the duplicates table.

The return format can be a simple json tree, a text repressentation that is mostly useful for debugging, and two formats supported by D3.js via Graph::D3

TODO

• [x] Needs to check bug visibility.
• [x] Needs docs
• [x] Need to remove unreachable vertices (not needed from json_tree)

EXAMPLES

bug tree format

force_directed_graph

https://gist.github.com/dylanwh/b2116b84142fa0c6c72c94335dce30d1