Remove remaining implicit jQuery imports

- Remove obsolete comments in `src/sidebar/test/bootstrap.js` about
  Angular using the full version of jQuery. This no longer applies.

- Remove the last few uses of the `$` global variable to reference
  jQuery and replace it with an explicit import. This will make it
  easier to identify the remaining parts of the app that are still using
  it.

src/annotator/plugin/test/bucket-bar-test.coffee

+$ = require('jquery')
+
 BucketBar = require('../bucket-bar')
 
 # Return DOM elements for non-empty bucket indicators in a `BucketBar`.

src/annotator/plugin/test/cross-frame-test.coffee

+$ = require('jquery')
+
 Plugin = require('../../plugin')
 CrossFrame = require('../cross-frame')
 { $imports } = require('../cross-frame')

src/annotator/test/sidebar-test.coffee

+$ = require('jquery')
+
 { default: events } = require('../../shared/bridge-events')
 
 Sidebar = require('../sidebar')

src/sidebar/test/bootstrap.js

 // Expose the sinon assertions.
 sinon.assert.expose(assert, { prefix: null });
 
-// Load Angular libraries required by tests.
-//
-// The tests for Client currently rely on having
-// a full version of jQuery available and several of
-// the directive tests rely on angular.element() returning
-// a full version of jQuery.
-//
-
-import jQuery from 'jquery';
-
 import 'angular';
 import 'angular-mocks';
 
-window.jQuery = window.$ = jQuery;
-
 // Configure Enzyme for UI tests.
 import 'preact/debug';