Remove flatten hack and use lodash

[emilgoldsmith]

Expected Behavior

We shouldn't have hacky global prototype overrides ;), also the spec has changed from flatten to flat. (which was the reason no babel plugin was in yet, it was still in discussion)

Current Behavior

Emil (yeah that's me) implemented hacky global prototype overrides, see here

gazelle-server/src/index.js

Lines 1 to 7 in bd025e0

	// Emil hacking because he can't find a babel plugin that does it for some reason
	if (!Array.prototype.flatten) {
	// eslint-disable-next-line
	Array.prototype.flatten = function flatten() {
	return this.reduce((acc, cur) => acc.concat(cur), []);
	};
	}

Possible Solution

Just remove the prototypes hacks and use lodash