Hammer.js is used to facilitate resizing the sidebar by dragging the sidebar
button. For this use case, we can avoid a dependency which is not actively
maintained, get more visibility into what the code is doing and make testing
easier by using a small amount of our own code.

In the process the tests for drag resizing in the sidebar were refactored to
avoid referencing private fields of `Sidebar` and mocking `getComputedStyle`.

The user-facing behavior should be unchanged.