Migrate remaining website tests to JavaScript

[rolyp]

Perhaps not trivial, because the current PureScript helpers return Aff, which makes them hard to chain together in JavaScript. We could probably migrate these to return Promise, and then migrate the 2 remaining PureScript website tests to JS, but might be easier todrop the PureScript helpers entirely and replace by JavaScript equivalents. Depends how much automation we imagine doing that might be nicer to have in PureScript.