Add support for batch assignment overrides via web extension

[samandmoore]

Summary

It's common for users of the web extension to edit multiple split assignments at once. Up until now, this requires a separate API call for each assignment. This is inefficient, so we're introducing a new endpoint that can process a batch of assignment overrides all at once.

Other Information

To take advantage of this, the web extension will need to be updated, and probably the JS client as well.

In the meantime, the old endpoint will keep on working 👍🏻

/domain @Betterment/test_track_core /no-platform

~NOTE: i'm having issues installing libv8 + therubyracer locally. it might be caused by xcode 12, but i'm not sure. so there may be failing tests right now. i wrote them up but wasn't able to run them locally 😬~ I've got a workaround, use mini_racer locally 😄

[nanda-prbot]

Needs somebody from @Betterment/test_track_core to claim domain review

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[aburgel]

<< domain tafn!

[nanda-prbot]

@samandmoore needs to incorporate feedback from @aburgel. Bump when done.

_{HOW TO: Resolve Feedback}

[samandmoore]

bump!

[nanda-prbot]

Needs @aburgel to provide domain review

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[aburgel]

domainlgtm!

[nanda-prbot]

Approved! :dancers: :nut_and_bolt: :taco: