Handle multi-plugin submissions elegantly

Right now we cap web submissions at 5 plugins to prevent tests from timing out (see [vision issue #91](https://github.com/brain-score/core/issues/91)). This is a burden on users, and additionally will still occasionally time out.

Long term, we would like to handle larger submissions with some considerations for what sorts of bulk submissions we allow.

* [ ] Consider batching large submissions into either
* multiple PRs
* separate jobs on Travis

* [ ] Consider whether we want to add checking to only allow certain kinds of bulk submission