Use external aritfacts. Fix rekall gui and hunts.

- This commit removes all of the non-grr specific artifacts in favor of using
  the forensicartifacts.com repo from github.  See
  https://github.com/google/grr-doc/blob/26983a4b3b8faac286e7ea445f756f3680b33b29/user_manual.adoc#artifacts
  for more details. Note that if running from src, you will need to do a "python
  setup.py build" for things to work
- Fix rekall gui rendering
- Fix problem with GRR labels dropdown
- Make sure aritfacts uploaded through the artifact manager are validated
- Remove broken webplugins flow, replaced with artifacts
- Clean up of collections code, added more stats
- Fix email cc'ing for hunt approvals
- Re-instate server-side compatibility to fix hunt problems
  https://github.com/google/grr/issues/83