#317 Created AssignmentStore file.

I haven't manually tested this nor have I written the Jest tests for this yet, but I wanted to submit this first just break up the diffs into smaller pieces. I'll probably write the Jest test next.
We're moving the stores to use standard ES6 Promises now (instead of jQuery promises), since that's part of the latest JS language specification. Check out the other stores for examples. This gets us closer to removing our need for jQuery.