Require core before itemset

https://github.com/dimagi/Vellum/commit/e67377561ebb7f50126b27154d1db00524d7a5e7

When itemset is not enabled (aka HQ) then vellum fails to load.

It's a little difficult to go through the compiled JS, but it errors on this line trying to use vellum before it's been defined: return t.vellum.plugin("itemset", {}, {getSelectQuestions: function() {