Add specs for syntax_highlight JS

Also makes it work when given a parent element containing a
`.js-syntax-highlight` element so for dynamically-added things like
notes or Markdown previews, we can more accurately target just the
element we care about.

app/assets/javascripts/syntax_highlight.coffee

+# Syntax Highlighter
+#
 # Applies a syntax highlighting color scheme CSS class to any element with the
 # `js-syntax-highlight` class
 #
@@ -6,7 +8,13 @@
 #   <div class="js-syntax-highlight"></div>
 #
 $.fn.syntaxHighlight = ->
+  if $(this).hasClass('js-syntax-highlight')
+    # Given the element itself, apply highlighting
     $(this).addClass(gon.user_color_scheme)
+  else
+    # Given a parent element, recurse to any of its applicable children
+    $children = $(this).find('.js-syntax-highlight')
+    $children.syntaxHighlight() if $children.length
 
 $(document).on 'ready page:load', ->
   $('.js-syntax-highlight').syntaxHighlight()

spec/javascripts/syntax_highlight_spec.js.coffee

+#= require syntax_highlight
+
+describe 'Syntax Highlighter', ->
+  stubUserColorScheme = (value) ->
+    window.gon ?= {}
+    window.gon.user_color_scheme = value
+
+  describe 'on a js-syntax-highlight element', ->
+    beforeEach ->
+      fixture.set('<div class="js-syntax-highlight"></div>')
+
+    it 'applies syntax highlighting', ->
+      stubUserColorScheme('monokai')
+
+      $('.js-syntax-highlight').syntaxHighlight()
+
+      expect($('.js-syntax-highlight')).toHaveClass('monokai')
+
+  describe 'on a parent element', ->
+    beforeEach ->
+      fixture.set """
+        <div class="parent">
+          <div class="js-syntax-highlight"></div>
+          <div class="foo"></div>
+          <div class="js-syntax-highlight"></div>
+        </div>
+      """
+
+    it 'applies highlighting to all applicable children', ->
+      stubUserColorScheme('monokai')
+
+      $('.parent').syntaxHighlight()
+
+      expect($('.parent, .foo')).not.toHaveClass('monokai')
+      expect($('.monokai').length).toBe(2)
+
+    it 'prevents an infinite loop when no matches exist', ->
+      fixture.set('<div></div>')
+
+      highlight = -> $('div').syntaxHighlight()
+
+      expect(highlight).not.toThrow()