Merge branch 'fix-fullscreen-preview-in-milestones' into 'master'

Fix Markdown preview not working in Edit Milestone page

### What does this MR do?

This MR removes the automatic Zen Mode URL update when a hash is present and makes Markdown preview work again in the Edit Milestone page. I think the intent was to make it possible to link to a full-screen edit with a URL (e.g. http://foo/bar/issues/1/edit#fullscreen), but I don't this has ever worked. Perhaps a future MR can support this.

### Why was this MR needed?

A JavaScript error would be seen in the Milestone Edit page:

```javascript
Uncaught error, unrecognized expression: $('.zennable input[type=checkbox]##md-preview-holder')
```

In the Milestone Edit page, apparently the use of the hash `#md-preview-holder` causes the Zen Mode JavaScript to attempt to parse the hash for the `fullscreen_` prefix and look up the checkbox based on a unknown ID.

### Are there points in the code the reviewer needs to double check?

If we want to keep this hash update, an alternative fix is to add an `if` check in the beginning of `checkboxFromLocationHash`:

```coffeescript
if (window.location.hash.indexOf('#' + ZenMode.fullscreen_prefix) == -1)
  return null
```

### What are the relevant issue numbers?

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

See merge request !711

CHANGELOG

@@ -2,6 +2,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.12.0 (unreleased)
   - Refactor permission checks with issues and merge requests project settings (Stan Hu)
+  - 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)
   - Remove Rack Attack monkey patches and bump to version 4.3.0 (Stan Hu)

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()