This is the first of what is likely to be many steps to move our

codebase towards a more modern feel of using promises and

async/await for asynchronous operations. In this change,

BaseCollection.fetch has been changed, along with everything that

actually uses that including similar methods inside

ResourceCollection. The old callback usage is still available through

a helper which will wrap the returned promise, but causes a warning to

be logged to the console.
This also tweaks to our browserslist config in order to ensure that

async/await won't get polyfilled out due to niche browsers

(primarily on mobile) that we really don't care about supporting.

Ran js-tests.
Manually tested data population on the review dialog and post commit

  views.