This change converts the rest of the code in js/utils to ES6 (at least that

which makes sense to do--there are a few files where there's no benefit). The

actual code has gotten shorter, but the number of lines is bigger because I

fleshed out and updated the format of our docstrings.
This also removes some now-unused functionality to linkify text in Markdown

format (since we no longer do any Markdown rendering on the client side).
If you want a good example of why this is awesome, check out the new

implementation of _.bindCallbacks.

Ran js-tests.