Create management command to edit course's tabs

@chrisndodge @jinpa @sefk  Hey Chris -- could you take a look at this small command. Sef thought you were the right guy for this level of muck-with-data code.

This is a small script that our ops people could use to adjust the tabs per course, which apparently is quite common. Currently they use import/export, so this command both easier and more reliable. That said of course, it's a power tool to be used with care. If the cost/benefit makes sense, we could upgrade this to a real GUI driving the same sort of underlying logic.

Jane tested an earlier version with the same core logic on a real imported class and it seemed to work.

Jane: could you try --insert and then click around a bit to see that the class still works? I think we only tried --delete this morning (and of course I'm happy sit next to you and we can test it together).