Fix tracebacks

[mmerickel]

So this turned into a bit of a rabbit hole cleaning up a lot of cruft in the toolbar with regard to exceptions.

1. Tracebacks are now tied to the per-request toolbar object one-to-one. A request may have one traceback. Previously they actually stuck around for the entire lifetime of the app instead of being collected by the max_request_history setting.

2. The routes for exceptions are standardized to look similar to the SQLA ajax routes. For example, /{request_id}/exception instead of /exception?token=...&tb=... and /{request_id}/exception/execute/{frame_id}?cmd=... instead of /exception?token=...&tb=...&frm=...&cmd=....

3. Fixed the url generation for the traceback panel link at the bottom of the traceback... it was actually empty previously - it got lost somewhere along the way.

4. /favicon.ico is no longer specially handled.. it's just part of exclude_prefixes like anything else that you want to exclude.

5. request.pdtb_history is available for toolbar requests (mostly ajax requests or panel rendering).

6. Removed the unused history predicate.

7. URL generation was broken in the debugger.js but that's fixed now so the execute/source buttons work in tracebacks.