Prefer Object.assign to $.extend to merge objects

[guanw88]

Although Object.assign and $.extend have some differences (https://stackoverflow.com/questions/38345937/object-assign-vs-extend), the should not apply to us, as 1) we do not use the deep: true option in these methods, and 2) we are always merging in an empty object ({}), so it should never be undefined.

[diclophis]

this code appears to replace usage of $.extend with Object.assign correctly, however I would like to defer final approval to additional persons with more experience in these arts

[shirish-pampoorickal]

LGTM!

[jarednielson]

LGTM