Fix A/B test timing bug

[wlycdgr]

conf.save.enable_human-web is set during startup, which is intercepted by the dispatcher, which calls setupABTest, which calls setupHubPromoABTest. Because the call to setupABTest in the dispatcher.on handler is not then()-chained to an abtest.fetch call, setupHubPromoABTest may get called before the A/B tests have been fetched for the first time. The absence of either hub_midnight or hub_plain on the still-empty abtest.tests object would then have been incorrectly interpreted to mean that the user should be placed in the remaining test bucket.

I initially fixed this by putting the setupABTest call in the dispatcher.on handler in a then() clause of an abtest.fetch call, but I prefer the current fix because it avoids making an extra call to the A/B test server on every startup.

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[fcjr]

Looks to be working as expected