Allow requesting the unrendered dot graph from the graph endpoint

When api/graph.{dot,gv} is requested the raw string for dot gets returned.
This allows client-side rendering rather than server-side. It also makes
the optional dependency on graphivz for dot unnecessary to use the graph
endpoint.

system/t12_api/graph.py

@@ -15,3 +15,7 @@ class GraphAPITest(APITest):
         resp = self.get("/api/graph.svg")
         self.check_equal(resp.headers["Content-Type"], "image/svg+xml")
         self.check_equal(resp.content[:4], '<?xm')
+
+        resp = self.get("/api/graph.dot")
+        self.check_equal(resp.headers["Content-Type"], "text/plain; charset=utf-8")
+        self.check_equal(resp.content[:13], 'digraph aptly')