Return more information for bugs scanned from test case comments

Return the type and prefix as well in the bugs array of a test case. The
name is a bit misleading nowadays since not all IDs are necessarily
bugs, but keeping the name for compatibility.
Use same field names in hashify_test_session

Changed the report level field names in hashify_test_session to match
those used in other JSON endpoints. Will then replace the whole hashify
thingie with existing as_json methods.
Get rid of ReportExporter.hashify_test_session

The hashify_test_session was pretty much duplicating existing as_json
methods. Removed it completely, made the as_json methods more
configurable (since they already take options hash), and added a
temporary method for mapping the JSON data to the same format that has
been delivered to QA Dashboard so far. At some point QA Dashboard is
updated as well and this temporary mapping can then be removed (because
QA Dashboard update will most likely break the existing installations
anyway).