[bug] nivturk plugins sometimes fail to load

[Sam Zorowitz]

During data collection, an online participant reported to @seanmallan that they experienced a technical issue upon completing an experiment. Specifically, the participant was redirected to a blank screen. The participant looked for an error message and found Uncaught ReferenceError: redirect_success is not defined. In other words, the nivturk-plugins silently failed to load!

As a stopgap, @szorowi1 hard-coded the nivturk plugins into the experiment.html file. That is, @szorowi1 made it such that the redirect functions could never fail to be loaded. Afterwards, far fewer participants sent @seanmallan complaints.

This suggests one of two changes should be made:

1. The redirect functions in nivturk-plugins.js should be hardcoded into experiment.html.
2. The experiment cannot proceed until nivturk-plugins.js is successfully loaded.

@szorowi1 will look into the viability of (2), otherwise will proceed with (1).