Replace $.each with _.each

In a few cases the $.each was doing something imperatively that was
terser and easier to understand by using a different Underscore method,
so a few of these I rewrote.

Some code was using the fact that jQuery sets `this` in the callback to
be the item; I rewrote those to use an explicit parameter.

Some code was using $(some selector).each(callback). I converted these
to _.each($(some selector), callback).

One function, ui.process_condensing, was written to be a jQuery $.each
callback despite being in a totally different module from code using it.
I noticed this and updated the function's args.

(imported from commit bf5922a35f257c168cc09ec1d077415d6ef19a03)

static/js/debug.js

@@ -28,14 +28,14 @@ function print_elapsed_time(name, fun) {
 
 
 /* An IterationProfiler is used for profiling parts of looping
- * constructs (like a for loop or $.each).  You mark sections of the
+ * constructs (like a for loop or _.each).  You mark sections of the
  * iteration body and the IterationProfiler will sum the costs of those
  * sections over all iterations.
  *
  * Example:
  *
  *     var ip = new IterationProfiler();
- *     $.each(myarray, function (idx, elem) {
+ *     _.each(myarray, function (elem) {
  *         ip.iteration_start();
  *
  *         cheap_op(elem);