Fix Markdown preview not working in Edit Milestone page

Closes #1687
Closes https://github.com/gitlabhq/gitlabhq/issues/9325

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.12.0 (unreleased)
+  - Fix Markdown preview not working in Edit Milestone page (Stan Hu)
   - Add web hook support for note events (Stan Hu)
   - Disable "New Issue" and "New Merge Request" buttons when features are disabled in project settings (Stan Hu)
   - Allow to configure location of the `.gitlab_shell_secret` file. (Jakub Jirutka)

app/assets/javascripts/zen_mode.js.coffee

 class @ZenMode
-  @fullscreen_prefix = 'fullscreen_'
-
   constructor: ->
     @active_zen_area = null
     @active_checkbox = null
@@ -23,7 +21,7 @@ class @ZenMode
       if checkbox.checked
         # Disable other keyboard shortcuts in ZEN mode
         Mousetrap.pause()
-        @udpateActiveZenArea(checkbox)
+        @updateActiveZenArea(checkbox)
       else
         @exitZenMode()
 
@@ -32,14 +30,11 @@ class @ZenMode
         @exitZenMode()
         e.preventDefault()
 
-    $(window).on 'hashchange', @updateZenModeFromLocationHash
-
-  udpateActiveZenArea: (checkbox) =>
+  updateActiveZenArea: (checkbox) =>
     @active_checkbox = $(checkbox)
     @active_checkbox.prop('checked', true)
     @active_zen_area = @active_checkbox.parent().find('textarea')
     @active_zen_area.focus()
-    window.location.hash = ZenMode.fullscreen_prefix + @active_checkbox.prop('id')
 
   exitZenMode: =>
     if @active_zen_area isnt null
@@ -51,17 +46,3 @@ class @ZenMode
       window.scrollTo(window.pageXOffset, @scroll_position)
       # Enable dropzone when leaving ZEN mode
       Dropzone.forElement('.div-dropzone').enable()
-
-  checkboxFromLocationHash: (e) ->
-    id = $.trim(window.location.hash.replace('#' + ZenMode.fullscreen_prefix, ''))
-    if id
-      return $('.zennable input[type=checkbox]#' + id)[0]
-    else
-      return null
-
-  updateZenModeFromLocationHash: (e) =>
-    checkbox = @checkboxFromLocationHash()
-    if checkbox
-      @udpateActiveZenArea(checkbox)
-    else
-      @exitZenMode()