Switch report/index to use json_dump

These changes make the report/index view use json_dump as input, which is the raw JSON output from the stackwalker. This is a necessary step towards getting rid of the pipe dump stackwalker output.

This is split into three parts:

1. Changing utils.parse_dump to convert the pipe dump into the same schema as the json_dump. This way the later change to use json_dump is much simpler, and we don't have to maintain two code paths for backwards-compatibility.
2. Refactor bits of parse_dump out (like the vcs_mapping handling) so we can use them against the json_dump as well.
3. Switch to using json_dump.

r?@peterbe